Four mutagenicity tests of alpha-Ni3S2 were performed: the Ames test on five Salmonella typhimurium strains, HPRT test on V79 cells, in vitro chromosomal aberrations on human lymphocytes and the in vivo micronucleus test in mice. The in vitro tests were carried out without metabolic activation. (i) The Ames test (5-1500 micrograms/plate) demonstrated no mutagenic activity, thus confirming previous observations by other authors. (ii) The HPRT test was carried out under standard conditions (3 h exposure) with alpha-Ni3S2 concentrations from 30 to 1000 micrograms/ml. No significant difference was observed with the control cells. Ultrastructural examination revealed alpha-Ni3S2 binding to the cell membrane. Very few particles were found in the cytoplasm but not in the nucleus. (iii) In vitro metaphase analysis in human lymphocytes were performed after exposure to total alpha-Ni3S2 suspension and to the soluble fraction in culture medium with or without fetal calf serum (FCS) (3-100 micrograms/ml, 24 h exposure). Nickel concentrations in the soluble fraction were determined by electrothermal atomic absorption spectrometry. A clastogenic effect of alpha-Ni3S2 became evident under all experimental conditions with a significant additional increase of chromosomal aberrations in 20% FCS complemented medium. No difference was observed between total suspension and soluble fraction. (iv) The micronucleus test confirmed the clastogenic effect of alpha-Ni3S2 in vivo after administration of 250 mg/kg (i.p.). This test revealed a clear increase of micronuclei frequency in polychromatic erythrocytes 24, 48 and 72 h after the treatment. In addition, we observed a statistically significant decrease in the number (%) of polychromatic erythrocytes after 24 and 48 h exposure. The soluble, i.e. cell-entering, fraction seems to play a great part in the clastogenic effect, as has also been shown with other test methods by other authors.