The primary immune response to SRBC in BALB/c mice was depressed when they were injected with a fraction (FAd) obtained from Trypanosoma cruzi epimastigotes grown in LIT medium. Plaque-forming cell (PFC) number was 50% less than controls when FAd was injected i.v. 15 min before antigen in doses ranging from 70 microgram up to 400 microgram of protein. Similar depression was observed when 100 microgram FAd was injected up to 6 h before antigen. There was no shift in the peak response to SRBC, neither was depression detected, when a total of 100 microgram FAd protein was given in 20 microgram amounts twice a day before immunization. Mice injected with FAd fraction only showed no increase in background PFC. Both secondary IgM and secondary IgG PFC were depressed when FAd was given before the boosting injection. However, only IgG PFC were depressed when FAd was injected before the priming dose. The delayed-type hypersensitivity reaction to DNFB was depressed when animals were injected either during the 3 days after sensitization or with a single dose of 100 microgram of protein of FAd on day of challenge. Bone marrow colony-forming units in spleens of mice injected with FAd were depressed and nodules in the treated animals were smaller than in controls. We conclude that FAd affects humoral and cell-mediated immune responses by interfering with cell division at some stage of the cell cycle.

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