Oestrogen receptor (ER) concentrations in breast tissues are almost universally expressed in relation to total soluble protein, though the latter does not fully correct for gross variations in receptor concentration due to variations in tissue cellularity. It was considered possible that the concentration of a specific glycolytic enzyme might be a better index and reflection of tumour cellularity. Measurements of the concentrations of lactate dehydrogenase (LDH), oestrogen receptor and total soluble protein and estimates of tumour cellularity were therefore performed on 98 breast tissues (80 breast cancers, 18 benign tissues). Cellularity and the concentrations of oestrogen receptor, lactate dehydrogenase and total soluble protein were significantly higher in breast cancers than in the benign tissues. The concentration of oestrogen receptors (positives only) was, as expected, related to tissue cellularity (correlation coefficient, r = 0.35). The concentrations of both LDH and total soluble protein were also both strongly related to tissue cellularity (correlation coefficients r = 0.67 and 0.75, respectively), and to each other (r = 0.78). These results suggest that (1) total soluble protein concentration is a good index of cellularity and (2) there is unlikely to be any additional value in expressing receptor concentrations relative to LDH since the latter appears to be a 'typical' soluble protein.