Investigations of the mutagenicity of hycanthone methanesulfonate (HCT) in mammals have led to varied results. To avoid ambiguous interpretation of significant results previously reported in two dominant-lethal studies of HCT a nested analysis of variance was performed on the number of corpora lutea per pregnant female, the number of implantations per pregnant female, average preimplantation losses, the number of dead implants per pregnant female, and the number of live implants. The analysis showed that variability among males and females was not responsible for the reported significant effects and consequently established the validity of the previous studies. Based on a comparison of the mammalian mutagenicity work performed utilizing hycanthone, pharmacokinetic factors are thought to be responsible for the differences seen; consideration of these factors in the future may lead to fewer false-negative results in mammalian systems.