Phagocytic indices of 17 nickel compounds were measured in vitro in monolayer cultures of rat peritoneal macrophages. The macrophages were exposed for 1 h at 37 degrees C to particles (1.5 micrometer median diameter) of the nickel compounds, at concentrations of 10 microgram/ml of medium (2 microgram/cm2 of monolayer). Phagocytic induces (i.e., the percentages of macrophages with one or more engulfed particles) ranged from 69% (NiO) to 3% (amorphous NiS). In order to decreasing phagocytic indices, the 17 nickel compounds were ranked as follows: NiO greater than Ni4FeS4 greater than NiTiO3 greater than NiSe greater than alpha Ni3S2 greater than Ni greater than Ni5As2 greater than NiS2 greater than NiFe alloy greater than NiSb greater than Ni11As8 greater Ni3-Se2 greater than beta NiS greater than NiTe greater than NiAs greater than NiAsS greater than amorphous NiS. Rank-correlation (P less than 0.03) was observed between the phagocytic indices of the nickel compounds and their dissolution half-times in rat serum. Nickel subsulfide, alpha Ni3S2, was a notable exception to the general concordance between phagocytic indices and dissolution half-times: alpha Ni3S2 was avidly phagocytized by macrophages, yet it had one or the shortest dissolution half-times. Preliminary results of carcinogenesis tests of 14 of the nickel compounds do not indicate significant rank-correlation between the phagocytic indices of the nickel compounds and the sarcoma incidences at 1 yr after i.m. administration of the compounds to rats.