In vivo immune response is potentiated after orchiectomy. This is due to a change in serum immunomodulatory properties following castration. This was examined by testing the PHA and Con A mitogen responses of spleen cells and thymocytes from intact rats in the presence of media containing serum from individual sham operated or castrated male rats. Large differences in blastogenic response were found between individual sera. However, neither fresh nor stored castrate serum supplemented media potentiated spleen cell or thymocyte mitogen response compared to control serum supplemented media. Mitogen response was greater in the presence of fresh rather than stored sera for both rat groups.