We studied over 17,000 peripheral blood counts, accumulated during surveillance from 1940 through 1975, from a cohort of 459 benzene-exposed workers. Linear regressions demonstrated significant decreases in white and red cell counts, as well as hemoglobin, for workers exposed during the 1940's, without persistent trends over the ensuing 25 years. Strongly positive correlations were observed between these blood count fluctuations and fluctuations in retrospective estimates of benzene exposures for this plant for the earlier periods of surveillance (mean estimated exposure 1940 to 1948, 75 ppm), but not for later years (mean estimated exposure 1948 to 1975, 15 to 20 ppm). These data suggest substantial limitations of hematologic examination of populations to detect abnormalities in populations currently exposed to benzene. The data also demonstrate a novel approach to the biological validation of exposure estimates based upon limited industrial hygiene and historical record data.