In the classroom air on a primary school polychlorinated biphenyls were present in a wide range extending to 10.220 ng/cbm; in a side room even 19.550 ng/cbm were measured. The school was closed down, a flame retardant paint was detected to be the main source of this contamination. 130 differently exposed persons gave blood specimens for measurement of PCB concentrations. Compared with a comparative reference collective of equal age reported by the analysing laboratory, the PCB blood concentrations of 92 actual pupils were even lower. A comparison with five near-representative child collectives of South Germany did not show any obvious difference. The reference values of the German Human Biomonitoring Commission were exceeded by 4 of 92 pupils for PCB 138, by 6 pupils for PCB 153 and by 6 pupils for PCB 180. In a group of 9 teachers and 1 cleaning person the median values of reference collectives of equal age reported by the analysing laboratory were exceeded by 8 persons for PCB 138, 7 persons for PCB 153 and 8 persons for PCB 180. The 95. Percentiles of these reference collectives and the reference values of the German Human Biomonitoring Commission were not exceeded. In bivariate and multiple regression analyses the data of 92 actual pupils showed positive associations between blood levels of PCB and age as well as nursing period, a negative association between PCB blood concentrations and body-mass index and a weak influence by gender (boys had lower blood concentrations). In addition, the results of multiple regression analyses showed (with the exception of the analyses of PCB 180 without controlling for gender) a significant influence of PCB concentrations in indoor air on PCB blood concentrations. This association could not be detected in bivariate analyses, and in comparison to the mentioned main influence variables it was obviously weaker. But basing on the results of the multiple regression analyses one can calculate an increase of PCB blood concentration by 3% multiplicatively if PCB indoor concentrations increase by 1000 ng/cbm.