Recent investigations of certain enzymes as markers for predicting the response of breast tumours to hormonal therapy have neglected the possible differential contribution of cell subpopulation(s) within a solid tumour to enzyme activity. In this investigation, lactate dehydrogenase (LDH) and peroxidase activities in density-defined cell subpopulations from autotransplanted 7,12-dimethylbenz[alpha]anthracene-induced mammary tumours were determined. The effects of ovariectomy and subsequent oestrogen administration on the activity of these enzymes were also examined. Five cell subpopulations (cell bands) were routinely obtained from each mammary tumour. The highest LDH activity was found in cell band 4. The highest level of peroxidase activity was found in cell band 5. These two cell bands with high levels of enzyme activity consisted mainly of poorly differentiated cells. After bilateral ovariectomy, a significant (P less than 0.001) decrease in the level of LDH activity in cell bands 3, 4 and 5 was observed. The enzyme activity was reduced to 20, 2.1 and 12% of the preovariectomy levels respectively. Significant (P less than 0.05) decreases between baseline and postovariectomy peroxidase values were evident in each cell band. In the presence of oestradiol-17 beta, significant increases in the LDH activity of band 4 (P less than 0.001) and the peroxidase activity of band 5 (P less than 0.05) were observed. Our data suggest that, given the existence of multiple cell types in hormone-responsive tumour tissue, the actual cell subpopulation(s) responsible for any enzyme response may be a more precise indicator of hormone dependence.