1. Earlier studies have shown that Fe2+ transport into erythroid cells is inhibited by several transition metals (Mn2+, Zn2+, Co2+, Ni2+) and that Fe2+ transport can occur by two saturable mechanisms, one of high affinity and the other of low affinity. Also, the transport of Zn2+ and Cd2+ into erythroid cells is stimulated by NaHCO3 and NaSCN. The aim of the present investigation was to determine whether all of these transition metals can be transported by the processes described for Fe2+, Zn2+ and Cd2+ and to determine the properties of the transport processes. 2. Rabbit reticulocytes and mature erythrocytes and reticulocytes from homozygous and heterozygous Belgrade rats were incubated with radiolabelled samples of the metals under conditions known to be optimal for high- and low-affinity Fe2+ transport and for the processes mediated by NaHCO3 and NaSCN. 3. All of the metals were transported by the high- and low-affinity Fe2+ transport processes and could compete with each other for transport. The Km and Vmax values and the effects of incubation temperature and metabolic inhibitors were similar for all the metals. NaHCO3 and NaSCN increased the uptake of Zn2+ and Cd2+ but not the other metals. 4. The uptake of all of the metals by the high-affinity process was much lower in reticulocytes from homozygous Belgrade rats than in those from heterozygous animals, but there was no difference with respect to low-affinity transport. 5. It is concluded that the high- and low-affinity 'iron' transport mechanisms can also transport several other transition metals and should therefore be considered as general transition metal carriers.