The secretion of the rabbit seminal vesicle has been investigated by morphological and biochemical means. Apical regions of seminal vesicle epithelial cells show highly active protein synthesizing and releasing organells. The secretory proteins released are analysed by disc-electrophoresis and three different immunological procedures. There is evidence for the presence of an uteroglobin-like antigen in seminal vesicle secretion. Comparison with seminal plasma indicated that the uteroglobin-like protein is also present in this fluid. The immunological and electrophoretical identity of rabbit uteroglobin, obtained from the uterus, with "male uteroglobin" is obvious, but molecular-biochemical and biological identity awaits further clarification. The demonstration of uteroglobin-like antigen in the male as in the female points towards new aspects in reproductive and contraceptive research.