The in vitro carcinogenicity of sodium nitrite was examined. Addition of a high concentration of sodium nitrite (50 mM or 100 mM) to mass cultures of newborn hamster cells for 24 hr resulted in morphological transformation of the cells. The shortest time required for morphological transformation after this treatment was 21 days. Two of 5 transformed cultures produced progressively growing tumors when injected into young adult hamsters. These tumors were diagnosed as fibrosarcomas. In two control cultures of the same experimental groups, transformed cells appeared 10 weeks or more later than those in treated cultures. The chromosomes were analysed at several stages of transformation. In the early stages of transformation, chromosome number varied widely from near diploid to tetraploid, but later they became near diploid. Morphological alteration caused by sodium nitrite was also seen in a short-term assay, in which hamster embryonic cells (1 x 10(4) cells/60 mm dish) were treated and morphology was observed 8 days after the treatment.