Praziquantel, a new anthelmintic drug with antischistosomal and anticestodal properties, was tested in comparison with a placebo control and a 'positive control' with cyclophosphamide in mammalian test system in vivo for potential mutagenic effects. The test systems used and the tested doses of Praziquantel were: (1) Dominant lethal test on male NMRI mice, 12 mating periods of 4 days each, 1 X 1200 mg/kg BW by mouth; (2) Dominant lethal test on female NMRI mice, treatment during pre-estrus, 1 X 1200 mg/kg BW by mouth; (3) Micronucleus test on male and female NMRI mice, two doses with a 24-h interval and preparation of the femoral marrow 6 h after the second dose, 2 X 300 mg/kg and 2 X 600 mg/kg BW by mouth; (4) Spermatogonial test on the Chinese hamster, two doses with a 24-h interval and preparation of the spermatogonia 48 h after the second dose, 2 X 600 mg/kg BW by mouth. The 1200 mg/kg BW dose corresponded to approximately 1/2 of the LD50 after oral application in the mouse and about 40 times the therapeutic dose (1 X 30 mg/kg BW). The cyclophosphamide doses in the test systems were 1 X 200 mg/kg or 2 X 200 mg/kg BW by mouth. No indication was found of any mutagenic potency of Praziquantel. This agrees with the results of point-mutation tests done by other authors.