Prednisolone-immunosuppressed mice (ICR, 7-wk-old female) were each inoculated with 1 x 10(5) oocysts of Cryptosporidium parvum. Medication with azithromycin (400 mg/kg/day) or lasalocid (64, or 128 mg/kg/day) was started 13 h after inoculation and continued for 3 days. The number of oocysts discharged by each mouse was calculated on days 4-12 post-inoculation. Compared with non-medicated controls, oocyst production by the medicated mice was markedly reduced; some mice did not discharge oocysts and the remaining mice discharged less than 1/100 the number of oocysts of the control mice. These results indicate that both azithromycin and lasalocid have prophylactic or therapeutic activity against Cryptosporidium.