Summary.-A study has been made of the uptake of 63Ni2+ from its complexes with serum proteins and with the small molecules from a sterile autolysate of rat muscle by cells of the C57S/IP line of mouse dermal fibroblasts in vitro. Nickel from these complexes is incorporated intracellularly, the distribution being independent of the nature of the carrier. The cation is associated with all subcellular fractions, the largest amount being bound by the nuclei and the least by the microsomes. About half of the 63Ni2+ incorporated into the nuclei is located in the nucleoli.IT has been shown that the carcinogenic metals, cobalt, cadmium and nickel dissolve when incubated aseptically in horse serum and in homogenates of rat muscle with the formation of complexes of various biological ligands (Heath, Webb and Caffrey, 1969;Weinzierl and Webb, 1972). After solution in serum the dissolved cations are bound by both large (e.g. protein) and small (diffusible) molecules, whereas in muscle homogenates they are complexed almost entirely (e.g. 90 %) with compounds of low molecular weight. The products obtained on solution of nickel in a muscle homogenate in vitro are similar to those formed when implants of the metal dissolve in whole muscle in vivo, and thus it seems that these small molecular complexes may be the normal cation carriers that are involved in the transport of the cation during carcinogenesis. The cobalt serum complex obtained by dissolution of the metal in horse serum, however, also produces cytological changes in cultures of rat myoblasts similar to those seen in pre-malignant cells in vivo (Heath et al., 1969).To investigate whether carcinogenic metals, when bound by ligands of both high and low molecular weight, are incorporated by mammalian cells, a study has