The aim of this study was to investigate the antimutagenic effects of Spirulina (SP) on male and female mice by the dominant lethal test using cyclophosphamide (CP) as a mutagen. Animals of both sex were given SP orally at 0, 200, 400 or 800 mg/kg body weight (b.w.) for 2 weeks prior to starting the CP treatment. CP was i.p. injected daily for 5 days at 40 mg/kg b.w. For the male-dominant lethal test, each male was caged with untreated females per week for 3 weeks. For the female-dominant lethal test the above doses and schedule treatments were used and treated females were caged for one week with untreated males (1-2). On days 13-15 after breeding was |started all the females were evaluated for incidence of pregnancy, total corpora lutea, total implants and pre- and post-implant losses. In the male-dominant lethal test, the CP induced pre- and post-implant losses in untreated females were inhibited at all SP doses. In the female-dominant lethal test only post-implantation losses were prevented at the same doses. Semen examination of a separate group of mice showed that SP improved its quality. Our results illustrate protective effects of SP in relation to CP-induced genetic damage to germ cells.