Abstract The immunological relationship between intracytoplasmic A particles and B particles was investigated in order to determine whether A particles are the preformed cores of mature B particles. A serum prepared in rabbits against purified A particles (obtained from Leydig cell tumors) reacted, in immunodiffusion tests, to extracts from a variety of mouse tumors. It was found that tumors containing both A and B particles as well as those with only A particles contained A particle-associated antigens. Radioimmunoprecipitation and polyacrylamide gel electrophoretic (PAGE) techniques were used to identify those murine mammary tumor virus (MuMTV) proteins that were serologically related to the antigens of A particle. The MuMTV proteins were radiolabeled with either 14C-labeled amino acids in tissue culture or with 125I using the lactoperoxidase method. The solubilized viral proteins were tested with anti-A particle serum, or as controls, with antisera to MuMTV, MuMTV-gp55, and -p28, as well as normal rabbit and goat sera. Rabbit anti-MuMTV serum precipitated all the major polypeptides of MuMTV (gp55, gp36, p28, p18, and p12) when reacted with solubilized MuMTV proteins. However, rabbit anti-A particle serum precipitated only p28, p18, and p12. PAGE analysis of the MuMTV proteins precipitated by goat anti-MuMTV gp55 or by p28 sera showed only gp55 or p28; normal goat or rabbit serum did not precipitate detectable amounts of viral proteins indicating that the reactions were specific. These results suggest that A particles contain antigens which cross-react with the three major internal proteins of MuMTV and support the hypothesis that intracytoplasmic A particles are the preformed cores of B particles.