The activity of the anticoccidial drug, lasalocid, was tested against Toxoplasma gondii in cell cultures. Multiplication of parasites was inhibited by 0.05 mug/ml of lasalocid added to the cultures prior to adding the parasite inoculum, with the parasite inoculum, or after the parasites had penetrated the culture cells. Penetration of culture cells was inhibited when 0.05 mug/ml lasalocid was added with the parasite inoculum. Incubation of extracellular parasites in 0.5 mug/ml lasalocid had no effect on penetration or multiplication. Ormetoprim, sulfadimethoxine, and a combination of the 2 were less effective than lasalocid. Monensin exhibited an inhibitory effect in all experiments.