Tumorigenicity of isoniazid in Swiss and A strain mice was studied with three different doses (0.55, 1.1 and 2.2 mg/mouse/day). Mice were fed isoniazid by gastric intubation and killed when they appeared weak. Incidence of tumour ranged from 32 to 100%. Subsequently sex-specificity in isoniazid tumorigenicity was also studied as well as the effect of vitamin B complex or protein deficiency, and of transplacental exposure of Swiss strain embryos to isoniazid. It was observed that protein deficiency and transplacental exposure stimulated tumour incidence in mice treated with 0.55 mg of isoniazid. No sex specificity was observed in isoniazid tumorigenesis.