A suppression of the IgE antibody response to ovalbumin was obtained in susceptible mice infected with Paracoccidioides brasiliensis yeast cells a few days prior to immunization with the former antigen plus adjuvant. A direct relationship between the number of injected fungi and the suppressive effect was established. When infection with a pathogenic isolate of P. brasiliensis (Pb 18) was compared to a non-pathogenic isolate (IVIC Pb267), the IgE anti-ovalbumin response was reduced by both. A similar effect was observed if mice were injected with dead yeast cells prior to immunization. Two strains of mice with completely opposite susceptibilities to infection with Pb18 cells (B10.A--susceptible and A/SN--resistant) both showed suppressed IgE anti-ovalbumin antibody production when infected 3 days prior to immunization. Injection of both strains of mice with P. brasiliensis antigen on the same day as immunization also had the same suppressive effect. These results suggest that the suppression of IgE response to an unrelated antigen in experimental murine paracoccidioidomycosis could be due to antigenic competition or to a suppressive component present in P. brasiliensis cells.