The effect of silica treatment on the course of Babesia rodhaini infection was investigated in ICR mice pretreated with either a mixture of B. rodhaini parasitized red blood cell hemolysation-sonication (S) antigen and Propionibacterium acnes bacterin or P. acnes bacterin alone and not subjected to immunization. In the normal mice (non-immunized), the mean survival time was decreased by approximately 1 day when 5 mg per mouse of silica were intraperitoneally inoculated at 5 h before infection. In immunization with S antigen and P. acnes bacterin, all mice survived regardless of the dose of silica (15-25 mg per mouse) treatment. In contrast, in immunization with P. acnes bacterin alone, 3/10, 4/10 and 1/10 mice died of B. rodhaini, when inoculated with 15 mg per mouse, 20 mg per mouse and 25 mg per mouse of silica treatment 5 h before infection, respectively. These results suggest that peritoneal macrophages (M luminal diameter of), especially when activated non-specifically by P. acnes bacterin, play an important role in inhibiting multiplication of organisms at the early stage of infection and the mice immunized with S antigen were ready to quickly acquire an enhanced solid specific protection even if the function of M luminal diameter of was damaged by a high dose of silica treatment.