The need to screen potential chemical pollutants for mutagenicity has increased with the increasing volume of such materials being introduced into natural water. The present study demonstrates the utility of a fish, the guppy (Poecilia reticulata), as a model test system in which water-borne chemical mutagens may be assayed for dominant lethal effects. Mature male guppies were injected with three doses of triethylenemelamine (0.1, 0.2 and 0.4 mg/kg) in addition to a control sham treatment. Each male was subsequently mated to virgin females. In an alternative test, male fish were allowed to swim in triethylenemelamine solutions of known concentration for a period of 24 h prior to being mated to virgin female guppies. 10 days following matings, females were dissected, and numbers of live and dead embryos were recorded. Significant dose effects were demonstrated by analysis of variance techniques in both the injection and the emersion tests with the results showing higher percentages of dead embryos and lower total number of embryos with increasing doses of TEM.