A solid-phase radioimmunoassay utilising bind of 125I-labelled protein A to antibodies bound to virus adsorbed onto microtitre plates was shown to be suitable for detection of humoral immunity to Gross murine leukaemia virus (MuLV). The specificity of the reaction was shown by the fact that only homologous or closely related viruses effectively inhibited binding of antibodies to adsorbed virus. With this method a low level of spontaneous humoral immunity was demonstrated in sera from AKR/Crc mice, a strain with high concentrations of endogenous virus, whereas little or no anti-viral activity was found inCBA/H-T6Crc, a subline that does not appear to express MuLV.