Nucleus anomaly test in Chinese hamsters and in rats treated with isoniazid (INH) was carried out according to a standard protocol in two different laboratories. These comprised both short-term studies, in which the tests were performed on animals killed 6, 12, 24, 36, or 48 h after the second of two consecutive doses of 5, 25, or 125 mg/kg INH given at an interval of 24 h, and long-term studies in animals treated with 25 mg/kg INH thrice weekly for 12 weeks. As a rule, each group consisted of at least four animals, and 1000 cells from each animal were examined. In one of the laboratories, a slight, but statistically significant increase in the incidence of nuclear anomalies was observed in two experiments on animals sacrificed 24 h after treatment; in the majority of cases, however, the investigations yielded negative results. Two out of three long-term studies revealed a slight, but statistically significant increase in the incidence of nuclear anomalies.