Semen assessment and levels of serum and seminal plasma prolactin (PRL) were determined in 81 men. In subjects with both normal sperm concentrations and normal sperm motility, the levels of serum and seminal plasma PRL were 14.7 +/- 1.48 and 10.0 +/- 1.05 ng/ml (mean +/- standard error of the mean, SEM, P less than 0.05), respectively. This difference is not observed either in subjects with polyzoospermia with normal sperm motility, or in those with oligozoospermia. Serum PRL was higher in azoospermia and also in subjects with lower levels of seminal citric acid. Seminal plasma PRL was directly related to sperm motility (r = 0.70, P less than 0.01) and inversely related to sperm concentration (r = -0.42 P less than 0.05). Seminal PRL was increased in subjects with higher levels of seminal citric acid and decreased in subjects with lower levels of corrected seminal fructose. Serum and seminal plasma PRL did not change significantly in subjects with different concentrations of serum testosterone.