RIA of specific IgG in serum and epididymal fluids following systemic immunization of male rabbits with DNP‐BGG showed a mean IgG epididymal fluid/serum molar ratio of 4.0 times 10‐3, and in epididymal fluid a mean concentration of 3.8 times 10‐7moles/1 or the equivalent in the caudal lumen of 40 000 anti‐DNP IgG molecules/spermatozoon there. This concentration did not change after the distension of the duct seen in the first months after vasectomy nor very markedly in the first 6 days after androgen withdrawal brought by castration. But the mean luminal level of specific IgG did rise some ten‐fold in the cauda subjected to body temperature for 6 days. Consideration is given to the prospect that such levels of antibody could compromise fertility if this were directed against some component that is essential for sperm