Comparative studies of the interactions of plutonium, americium and curium with serum proteins from various species, with proteins isolated from the organic matrix of bone, and with some other proteins have all shown that plutonium is bound very much more firmly by protein than either americium or curium. Investigations of the sub-cellular distribu-tion patterns of these elements in liver and testes have shown that all three elements become associated with lysosomal structures and that different mechanisms of lysosomal uptake are involved for polymeric and monomeric plutonium, americium and curium. The implications of these findings are discussed in relation to the types of biological damage which has been observed in experimental animals.