Bioactive FSH and immunoreactive FSH were determined in 193 infertile men and in 23 men with proven fertility using the Sertoli cell aromatase bioassay for bioactive FSH measurement and a two-site fluoroimmunoassay for immunoreactive FSH measurement. Overall bioactive and immunoreactive FSH levels correlated well (r = 0.74, p less than 0.001) but were significantly different from fertile men (bioactive FSH: 6.2 +/- 0.3 U/l; immunoreactive FSH: 4.1 +/- 0.4 U/l) in patients with Klinefelter's syndrome (24.1 +/- 6.1; 26.9 +/- 3.0), non-obstructive azoospermia (25.1 +/- 4.3; 22.2 +/- 4.0), maldescended testes (12.5 +/- 4.6; 14.6 +/- 1.6), and patients with severe oligozoospermia (11.9 +/- 1.2; 11.2 +/- 1.0). Infertile men with moderate oligozoospermia (8.9 +/- 1.5; 8.0 +/- 1.1) and normal sperm counts (9.6 +/- 1.1; 7.6 +/- 1.0) had insignificantly elevated bioactive FSH and immunoreactive FSH levels. Bioactive to immunoreactive FSH ratios were significantly reduced in all patient groups except for patients with normal sperm counts when compared with fertile men. A considerable number of patients exhibited elevated immunoreactive FSH concomitant with normal bioactive FSH levels. We conclude that 1. determination of immunoreactive FSH suffices for classification of patients; 2. bioactive to immunoreactive FSH ratios are reduced in infertile men; 3. some men might secrete immunoreactive FSH with reduced bioactivity.