The effects of purified human sex hormone-binding globulin (SHBG) on androgen-sensitive cell proliferation were examined using a human prostatic cell line (LNCaP-FGC). Cells were grown for 5 days in medium supplemented with 10% charcoal-dextran-stripped human serum (10% CDHuS) and various concentrations of 5 alpha-dihydrotestosterone (DHT). In 10% CDHuS, without SHBG, the proliferative response of these cells to androgens was typically biphasic. At low androgen concentrations, cell yields were increased in a dose-dependent manner, reaching maximal levels at 0.3 nM DHT. However, at high androgen concentrations, cell proliferation was inhibited. Addition of purified human SHBG to the medium reduced the effectiveness of DHT on both phases of the proliferative response in a dose-dependent manner. These effects of SHBG appeared to be due primarily to the high affinity binding of DHT by SHBG. Proliferative responses induced by the synthetic androgen methyltrienolone (R1881), which binds poorly to SHBG, were not affected by added SHBG. Furthermore, analysis of the protein binding of DHT revealed that cell proliferation correlated best with the concentration of DHT not bound to SHBG. The presence of SHBG in the medium also altered the uptake and metabolism of DHT. LNCaP-FGC cells rapidly metabolized DHT to a polar glucuronidase-sensitive conjugate of DHT. In 10% CDHuS, LNCaP-FGC cells conjugated virtually all of the added DHT during the 5-day experiment. However, in medium containing SHBG, the SHBG-bound DHT remained unconjugated; more than 90% of the DHT initially bound to SHBG was present in the medium at the end of the experiment as unconjugated DHT. Uptake of radiolabeled DHT by cells was also inhibited by SHBG. In summary, these experiments provide evidence that 1) SHBG-bound DHT is not a signal for DHT-induced cell proliferation and 2) SHBG inhibits the uptake and metabolism of DHT by LNCaP-FGC cells.