Cryostat sections of unfixed human breast carcinomas were assayed for oestrogen receptor (ER) content using an isoelectric focusing method to separate the receptor-bound oestradiol. Adjacent sections from the same tissue block were stained so that the tumour content could be estimated by point counting and the ER concentration adjusted to compensate for variations in cellularity. Elastosis was also assessed. The results confirm a positive correlation between ER values and both cellularity and elastosis. The measurement of ER in cryostat sections is relatively simple and rapid, is applicable to small tissue samples, and permits histological identification of the nature and composition of the assayed sample. The method is directly applicable to oestrogen receptor analyses of breast carcinomas in clinical laboratories with facilities for cryostat microtomy.