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NTP Report on the Immunotoxicity of Chlorpyrofos in Female B6C3F1 Mice (Contact Hypersensitivity Studies) (CASRN: 2921-88-2)


The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program (NTP). The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by the NTP on March 2009 (see The findings and conclusions for this study should not be construed to represent the views of the NTP or the US Government.

NTP Report Number IMM95001


Introduction: Chlorpyrifos (CPF) was selected for evaluation as a sensitizing agent for contact hypersensitivity in mice. CPF is a commonly used broad range insecticide. Occupational exposure occurs primarily by inhalation or skin exposure. CPF is also marketed for home use fumigation for fleas increasing the risk of possible exposure of the general public. Topical exposure may cause slight eye irritation and prolonged exposure may cause skin irritation. CPF is organophosphate pesticide and is known to induce delayed polyneuropathy in man.

Design: Chlorpyrifos, Mol. Wt . 350.57, was obtained from NIEHS and was received on 16 Dec 1992 as an off-white irreqularly flaked solid Lot#MM890522-25, Dow Elanco). Dilutions were made in acetone which served as the vehicle. Three assays were used in this assessment, a primary irritancy study, to screen for toxicity and determine the Minimal Irritating Concentration (MIC) and the Maximal Non-Irritating Concentration (MNC) and two assays, the Mouse Ear Swelling Test (MEST) combined with a Local Lymph Node response after senstization and challenge and the Local Lymph Node Assay (LLNA), to test the dermal sensitizing potential oi CPF.

The design for the MEST combined with the LLNA is shown in the table below. Pre, 24 and 48 hrs post treatment ear measurements were taken and used to calculate the precent ear swelling. Measurements from animals dosed with varying concentrations of test article were compared to the CPF background control for significance.


Design for the MEST combined with the LLNA:

Group Description Designation Sensitized Challenge
1 Baseline Control BC Vehicle 3.0%
2 Dose 1 D1 0.3% 3.0%
3 Dose 2 D2 1.0% 3.0%
4 Dose 3 D3 3.0% 3.0%
5 Irritation Control C Vehicle 0.5% DNFB
6 Positive Control PC 0.25% DNFB 0.5% DNFB

The chemical concentrations and experimental groups to be used for the LLNA are shown in the table below. Beta counts from lymph nodes of animals dosed with varying concentrations of test article were compared to the vehicle controls for significance.


Chemical concentrations and experimental groups used for the LLNA:

Group Description Designation Challenge
1 Vehicle VH Vehicle
2 Dose 1 D1 1.0%
3 Dose 2 D2 3.0%
4 Positive Control PC 0.25% DNFB

Results: Morbidity occurred in mice dosed with concentrations of 10 and 30% chlorpyrifos. Thus 3% was used as the challenge concentration for the MEST studies and the high concentration in the LLNA. There were no biologically significant changes in body weight of test groups of animals as compared to vehicle groups during the course of these studies. In the MEST, sensitization with concentrations of 0.3, 1 .0 or 3.0% CPF followed by challenge with 3.0% CPF did not show a significant change in percent ear swelling at either 24 or 48 hrs post challenge. Although a slight increase in lymph node proliferation occurred in Chlorpyrifos exposed mice, the magnitude was not sufficient to indicate an immune mediated reaction.

Conclusion: No significant allergic contact hypersensitivity response was demonstrated in female B6C3F1 mice to Chlorpyrifos .

Report Date: September 1994


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