Zidovudine was present in the semen and serum of six patients with acquired immunodeficiency syndrome or the related complex who were receiving 200 mg of the drug orally every four to six hours. Mean semen zidovudine levels (as measured by a new radioimmunoassay) in samples collected 0.75 to 1.25 hours after oral dosing were 3.63 to 7.19 mumol/L. Levels in semen samples collected 3.0 to 4.5 hours after oral dosing were 1.68 to 6.43 mumol/L. These values are above the in vitro minimum inhibitory concentration for the human immunodeficiency virus type 1 (HIV-1). Mean serum concentrations at the early and late times after oral dosing were 0.22 to 3.07 mumol/L and 0.10 to 1.42 mumol/L, respectively. Ratios of semen/serum zidovudine levels ranged from 1.3 to 20.4. It is possible that a pH-dependent trapping mechanism, which has been described in the prostate for other antibiotics, was responsible for the relatively high semen levels observed.