SUMMARYChronic treatment of adult men with LHRH agonists causes a decrease in serum testosterone and 5α‐dihydrotestosterone to castrate levels. In the presence of such low levels of circulating testicular androgens, the concentration of serum LH measured by radioimmunoassay (RIA) sometimes remains normal or is only partially inhibited. In order to assess the biological activity of circulating LH, we have used the mouse interstitial cell assay. Blood samples were obtained from patients with prostatic carcinoma treated with the LHRH agonist [D‐Trp6] LHRH ethylamide in combination with the pure antiandrogen Flutamide (Euflex). While serum LH levels measured by RIA were only partially reduced from 2.2±0.3 (SEM) to 1.1±0.1 ng/ml after 3 months of therapy, bioactive LH was markedly inhibited from 0.43±0.04 to 0.030±0.007 ng/ml, thus causing the ratio of biologically active to radioimmunoassayable LH to drop from 0.26±0.03 to 0.03±0.01. In the same patients, serum testosterone levels were decreased from 3.91±0.51 to 0.14±0.05 ng/ml after 3 months of treatment. In patients treated for 6 months, the bio/immuno ratio was still reduced at 0.032±0.005. These data show a marked loss of LH biological activity during treatment of adult men with an LHRH agonist and an antiandrogen. The close parallelism observed between serum testosterone and bioactive LH levels suggests that the loss of biological activity of the gonadotrophin is mainly, if not exclusively, responsible for the inhibition of testicular androgen secretion observed during chronic treatment with