Nucleus anomaly test in Chinese hamster and in rat after treatment with isoniazid

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.