In vitro studies have shown that the Sertoli cell is the primary source of inhibin in the male. We measured immunoreactive inhibin with a new two-site immunoenzymatic assay in the plasma of 92 men: 40 normal men, 7 patients with germinal cell cancer after unilateral orchidectomy and 45 patients with the same disease following unilateral orchidectomy and subsequent chemotherapy based on cisplatin. Normal men had inhibin levels of 1.77±0.09 U/l × 10−3 (mean±sem). Seven patients after unilateral orchidectomy had inhibin concentrations within the lower normal range (1.23±0.22 U/l × 10−3). Forty-five patients were investigated in a cross-sectional study up to 102 months after completion of chemotherapy. Inhibin levels were within the normal range in 25 patients (1.76±0.114 U/l × 10−3); 18 patients had significantly lower inhibin levels (0.48±0.05 U/l × 10−3, p<0.005) when compared to patients after unilateral orchidectomy. Two patients had elevated inhibin levels (4.4 and 5.6 U/l × 10−3). The proportion of patients with normal and subnormal inhibin was not dependent on the time that elapsed after completion of chemotherapy or on the chemotherapy combination. There was no correlation between immunoreactive plasma inhibin and LH, FSH, testosterone or sperm count. The decrease in inhibin concentrations after chemotherapy may indicate long-term damage to Sertoli cells in some of the patients.