1. In guinea pigs, aminomethylpteroylglutamic acid (methotrexate) is capable of blocking the development of delayed skin hypersensitivity, the primary antibody response, and the specific febrile response to ovalbumin and diphtheria toxoid. The primary antibody response is more easily inhibited than is the development of delayed skin hypersensitivity.
2. The effect of methotrexate on immunologic responses depended upon the dose of methotrexate employed and the strength of the antigenic stimulus.