Abstract: An in vitro screening test for identifying mutagenic and genotoxic agents during spermatogenesis, comprising: (1) Culturing Xenopus testis explants in vitro in the presence of one or more suspected mutagenic and/or genotoxic agent(s); (2) Removing said one or more suspected agent(s) and continue culturing said Xenopus testis explants such that spermatogonia in said Xenopus testis explants undergo spermatogenesis; (3) Isolating sperm and/or one or more types of spermatogenic cells at various times during step (2); and (4) Determining the mutagenic and/or genotoxic affect of said agent(s). A novel medium suitable for culturing Xenopus testis explants. A novel method for culturing Xenopus testis explants in vitro such that cells at all stages of the spermatogenic cycle are produced comprising culturing Xenopus tests fragments in the novel medium under an atmosphere of air and at a temperature in a range of from about 20.degree. C. to about 24.degree. C.