Lead (Pb) in blood, bone, and semen was measured in 162 to 186 environmentally exposed men from Mexico City, aged 19- 48. Semen Pb was measured by inductively coupled plasma mass spectrometry, blood Pb by atomic absorption spectrometry and bone Pb by K X-ray fluorescence. Mean Pb levels in blood, semen, tibia (cortical) and patella (trabecular) bone were 12 microg/dl, 2.7 microg/l, 13 microg/g, and 20 microg/g, respectively. Semen Pb was determined by blood Pb and patella Pb. Determinants of higher tibia Pb were age, living near industry in which Pb is used, and a high occupational Pb exposure index. Higher patella Pb was predicted by age, higher traffic density near home, a high index of occupational exposure to Pb and a greater number of cigarettes smoked per day in the year prior to the study. Blood and bone Pb results are consistent with findings in other populations. Semen results provide new information on the semen-bone Pb relationship. Bone, especially trabecular one, proved to be a significant endogenous lead source for blood and semen burdens in reproductive aged men.