A solid-phase radioimmunoassay using 125I-protein A is described for the detection of antibodies to collagens of different types. The optimal conditions for the adsorption of collagen onto polystyrene microplates, then the incubations with the antiserum and finally with the 125I-protein A have been evaluated. The technique was applied successfully to antisera raised in rabbit, goat, guinea pig and mouse against human type I, II, III, IV, V and bovine type I, II, 1 alpha 2 alpha 3 alpha, X1-X7 collagens.