In mid-1977 a number of cases of infertility among male pesticide workers in California came to light. A description of this problem was published as a Preliminary Communication in The Lancet. A larger clinical-epidemiological study was undertaken to better understand the exposure-effect relationships involved. Of 142 non-vasectomized men providing semen samples, 107 had been exposed to 1,2-dibromo-3-chloropropane (DBCP) and 35 had not been exposed. There was a clearcut difference in both the distribution of sperm counts and the median counts between the exposed men and the not-exposed men, Of the exposed, 13.1% were azoospermic, 16.8% were severely oligospermic, and 15.8% were mildly oligospermic. Among the controls, 2.9% were azoospermic, none were severely oligospermic, and 5.7% were mildly oligospermic. Under workplace conditions, DBCP appears to have a selective effect on the seminiferous tubules.