In a longitudinal pilot study on the course of the PCB concentration in human milk during six months of lactation, some important PCB determinants could be studied in 23 women and their infants. PCB values were within the range of those found in the literature. Pearson correlation coefficients were calculated to investigate the association of the mean PCB concentration over the first half year of lactation with maternal parameters, such as age, height, weight, previous lactation period, education, occupation, residence, smoking, drinking and dietary habits as well as the infant parameters gestational age, birthweight and weight gain in the first six months of life. Since the PCB concentration on fat basis and the fat content of the milk were strongly inversely related, statistical analyses were carried out both on fat and on milk basis. In univariate analyses the PCB concentration on fat basis was most strongly associated with pre- versus post-pregnancy weight gain, age and occupation. After multiple regression analysis PCB concentration on fat basis remained significantly associated with weight gain changes and remained borderline (p less than 0.10) significantly related with occupation. The pre-pregnancy Quetelet Index of the mother (height/weight) and the estimated PCB content of the diet (primarily fish) were strongly correlated with the PCB concentration on milk basis. Only the Quetelet Index remained significantly related after multiple regression analysis.