The aim of this study was to determine the proliferative activity within the epithelial cells of the normal human breast in 122 patients (6 reduction mammoplasties and 116 fibroadenoma excisions) in relation to age and the phase of the menstrual cycle. Thirty three of the patients were on oral contraceptives and 33 were parous. Thin tissue slices were incubated with tritiated thymidine and processed for autoradiography. Other samples were fixed directly and prepared for histology. The labelling, mitotic and apoptotic indices (LI, MI and AI) were determined and all illustrated considerable variability. The labelling indices are significantly (P less than 0.05) influenced by both patient age and stage during the menstrual cycle and ranged from 0-11.5%. Maximum LI values were obtained on the 20.8th day of the cycle. A square root transformation of the data was used to reduce the skewness of the data to a more normal distribution. The square root of the LI declined by 0.22 per decade. The mitotic data showed similar significant (P less than 0.05) correlations against age and day of cycle with a peak on the 21.5th day of the cycle, a decline by 0.072 per decade and a range from 0-0.6%. The data for apoptotic cells were less clearly influenced by the stage of the menstrual cycle but showed a significant (P less than 0.5) decline with age. The AI in parous patients was significantly higher than that in non-parous patients. There was no significant effect of oral contraceptives on any of the parameters measured when age and stage of cycle were taken into account. The considerable variability in the data could not be fully accounted for by either technical factors, the age of the patients, or the day of the cycle. We conclude that proliferation is negatively related to age and is influenced by the menstrual cycle but that additional as yet unknown factors must account for a large part of the variability seen in the data.