To determine if oil- and gas-facility emissions prospectively measured as airborne sulfur dioxide and volatile organic compounds (VOCs) were associated with immunomodulation in neonatal beef calves, veterinarians collected samples from 325 calves from 60 western Canadian herds in the spring of 2002. Researchers assessed immune system effects by enumerating B-lymphocytes and specific T-lymphocyte subtypes (CD4, CD8, gammadelta, and WC1) in peripheral circulation using flow cytometry. They estimated in utero exposure for each calf from the earliest possible breeding date of the dam to the calving date, using air-quality data from passive monitors installed in pastures and wintering areas. Numbers of circulating CD4 and CD8 T-lymphocytes were 42% and 43% lower, respectively, in calves exposed to the highest quartile (> or = 0.378 microg/m3) of VOCs measured as airborne concentrations of benzene compared with calves exposed to concentrations in the lowest quartile (< 0.276 microg/m3). Similarly, the number of CD4 T-lymphocytes was 40% lower in calves exposed to VOCs measured as concentrations of toluene in the highest quartile (> or = 0.713 microg/m3), compared with calves exposed to concentrations in the lowest quartile (< 0.348 microg/m3). There was no statistically significant association between in utero exposure and numbers of B-lymphocytes and gammadelta and WC1 T-lymphocyte subtypes in calf blood samples.