Free L-carnitine was assayed in semen from patients with various andrologic diseases by an enzymatic-spectrophotometric method. Extremely low concentrations were found in semen from patients with agenesis of the vas deferens (31.8 +/- 18.8 nm/ml). Semen from men with vasectomies contains a higher concentration of free carnitine (105.0 +/- 8.1 nm/ml). When comparing these data with those of ten fertile control subjects (817.0 +/- 200 nm/ml), we assume that seminal free L-carnitine mainly originates from the epididymis. Semen from patients with azoospermia caused by testicular failure also contains a low carnitine concentration. In hypogonadotropic eunuchoids the gonadotropin treatment increased the low basal concentration. A correlation between semen carnitine content and sperm motility and number was also tested in 124 infertile patients. The results show a positive correlation between free L-carnitine and sperm count (r = 0.617; P less than 0.01), between free L-carnitine and sperm motility (r = 0.614; P less than 0.01), and between free L-carnitine and the number of motile spermatozoa per milliliter (r = 0.646; P less than 0.01).