Five pesticides (carbaryl, DDT, parathion, chlordimeform, and ametryne) were tested for their effects on the immunocompetence of Balb/c mice. All five of the test pesticides were observed to induce statistically significant suppression of the humoral immune response if administered orally, at near lethal doses, during an ongoing immune response. Ametryne and chlordimeform were also observed to have a suppressive effect on humoral immune competence if orally administered in a sufficiently large quantity, 1 LD50, at the time of, or prior to, immunization. Administration of 0.1 LD50 parathion per day, over an extended period of time, resulted in a statistically significant suppression of the humoral immune response.