Contamination by bacteria or fungi is a frequent problem with the in vitro isolation and propagation of protozoan parasites. We developed an antibiotic combination of broad anti-contaminant activity but minimal toxicity to protozoa. This anti-contamination cocktail consists of penicillin G (60 microgml(-1)), kanamycin (100 microgml(-1)), chloramphenicol (10 microgml(-1)) and flucytosine (50 microgml(-1)). It is well tolerated by African and South American trypanosomes, Leishmania spp., Giardia duodenalis, Entamoeba histolytica and Plasmodium falciparum, and it provides an effective means for the treatment of contaminated cultures. The anti-contamination cocktail meets numerous prophylactic applications in which sterile handling is impracticable or impossible. In particular, it facilitates the direct in vitro isolation of parasites from an infected host.