Cadmium uptake was studied in respiratory epithelial gill cells from rainbow trout (Oncorhynchus mykiss). The cells were adherent and grown as a monolayer on the bottom of culture dishes. Experiments were made in chloride-free Cortland buffer, except when Cl- was included as an experimental variable. Cadmium was mainly taken up in the cells as Cd2+, but CdCl2 uptake was also observed. Increase in the calcium concentration from 0.1 to 12.8 mM led to a small decrease in cadmium uptake. Zinc and lanthanum had no effect on the uptake of cadmium. In the presence of xanthate (10(-6) - 10(-4) M) cadmium accumulation in the cells increased indicating that a hydrophobic xanthato complex was taken up across the phospholipid phase of the plasma membrane.