Tubules isolated from the initial segment, caput and corpus regions of the rat epididymis were incubated for 4 h in medium containing rat IgG or bovine-serum-albumin (10 mg ml-1) and processed for light microscopy immunocytochemistry using gold-labelled anti-rat IgG and silver staining or peroxidase/anti-peroxidase techniques. Distribution of IgG was confined to the peritubular matrix only, with no detectable uptake into the epithelium or lumen. In tubules incubated with rat IgG coupled to gold particles as tracer and fixed for electron microscopy, the IgG-gold complex failed to penetrate to the base of the epithelium. When IgG-peroxidase conjugates were used, sparse enzyme activity was localized in small basal and apical vesicles and multivesicular bodies of the principal cells. However, this uptake could not be inhibited by the presence of a 30-fold excess of non-labelled IgG, and the same distribution of enzyme activity was also observed when tubules were incubated with a similar activity of horseradish peroxidase alone. These findings indicate that there is no detectable receptor-mediated transport of IgG across the epididymal epithelium, although non-selective transfer could occur via fluid-phase endocytosis.