Nuclear and cytoplasmic proteins of human female breast cancer were analysed by one-dimensional sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Oestrogen receptor and progesterone receptor expression was determined by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) method. The electropherograms were developed by silver nitrate staining and quantitative analysis was carried out by video densitometer using the software Gel-Pro Analyzer. Nuclear and cytoplasmic proteins of breast carcinomas and normal tissue differed both qualitatively and quantitatively. Nuclear polypeptides of 108, 53 and 48 kD as well as the 36 kD cytoplasmic polypeptide were specific for tumour samples, while the 51 kD nuclear polypeptide was detected only in normal tissue. Quantitative differences in band density were noted in the 32 kD nuclear polypeptide. This polypeptide was expressed in greatest concentration in infiltrating ductal carcinomas which also indicated the greatest oestrogen receptor gene expression. This relationship appeared to be statistically significant (p < 0.005). No correlations were evident between the 32 kD protein expression and the progesterone receptor gene expression in any of the tissue types examined, nor between the 32 kD protein and the patient's age or tumour grade.