Mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and antimutagenic effect of antioxidant stobadine (STB) were investigated by so called HGPRT/V79 system. Cells were treated by STB before, during and after MNNG-treatment. Our results showed that the highest antimutagenic effect of STB was observed if the drug was given as a pretreatment before exposure of cells to MNNG. This effect was not concentration-dependent within the framework of 1.5-9 mmol. All other combinations of MNNG- and STB-treatment led to the weaker but statistically significant decrease of 6-TGr mutations. Inhibition of proteosynthesis induced by methylxanthine pentoxifylline in the time of pre-MNNG-treatment removed completely antimutagenic effects of STB. In addition to mutagenicity assays, cytotoxicity of STB and combined effects of MNNG and STB were studied. Trypan blue exclusion and growth activity of influenced cells showed that the application of STB (1.5 mmol) before or after MNNG (0.5 microgram/ml) treatment had a similar toxic effect as MNNG alone. Application of STB during MNNG-treatment or pretreatment of cells with STB followed by combined treatment of cells by STB+MNNG statistically significantly decreased viability of cells. There are probably no relationships between the antimutagenic and the toxic effects of combined influence of STB and MNNG on V79 cells.