Immunoelectrophoretic analyses were used to compare the antigenic characteristics of sperm and fluids recovered via cannulae implanted in the rete testis and proximal vas deferens of living bulls. Ejaculated semen, seminal vesicle fluid and blood serum also served as antigen sources. Although each reproductive fluid contained some blood antigens, these antigens were most numerous in rete testis fluid. Certain antigens present in rete testis fluid, but not detectable in blood serum, were associated with testicular sperm; others, common to seminal vesicle fluid, may be secretory products or enzymes originating in the seminiferous tubules. Some antigens apparently are selectively resorbed, utilized, or altered within the epididymis since all nonblood antigens entering the epididymis in rete testis fluid were not detected in cauda epididymal plasma. Other antigens in cauda epididymal plasma common to seminal vesicle fluid probably are secretory products of the epididymis. Studies of saline soluble antigens released by sonication of testicular and epididymal sperm revealed that sperm undergo antigenic modification during their epididymal passage. Some antigens apparently are lost from sperm into the surrounding fluid, while others possibly modified, remain associated with the cells. Furthermore, sperm acquire new antigens presumed to be secreted by the epididymis. Certain antigens available in seminal vesicle fluid at ejaculation already are present on testicular as well as epididymal sperm.