We have studied the binding of [ I-iodo]androgen-binding protein (ABP) and of [3H]Δ6-testosterone photoaffinity-labelled ABP to receptors in the plasma membrane of rat epididymal cells in three ways: ABP binding to a Triton X-100-solubilized membrane extract, ABP binding to isolated epithelial cells in suspension and autoradiography of segments of dissected epididymides after in-vitro intraluminal injection of labelled ABP. The binding of iodinated ABP to the receptor was similar to that of photoaffinity-labelled ABP in gel filtration. The ABP-receptor complex was eluted from Superose 6 gels as an aggregate, with a molecular mass of 2000 kDa. It was separated into two peaks by sucrose gradient ultracentrifugation, with respective sedimentation coefficients of 18.4 and 9.0s. The activity of the receptor (ABP-binding capacity/mg protein) was tenfold higher in the caput than in the cauda. The binding of ABP to the receptor was pH dependent, being almost abolished at pH <4. The binding at 4°C of photoaffinity-labelled ABP to epithelial cells corresponded to two types of binding sites. The numbers of high-affinity and low-affinity sites per cell were 1600 and 7700 respectively; the association constants of these sites were 67.9 and 2.8litres/ nm respectively. The binding was decreased by treatment of the cells with trypsin or incubation in the presence of ED TA. The binding in vitro of labelled ABP to the epididymis epithelium reached a maximum after about 20 min at 4°C. In the autoradiographic study the tracer was found to be closely associated with coated pits, coated vesicles, endosomes and pale multivesicular bodies. Treatment of rats with cycloheximide significantly reduced the uptake of the tracer. Perfusion in vitro of epididymides with chloroquine produced a fourfold increase of the tracer in endosomes and multivesicular bodies.