ER and PgR concentrations were assayed in primary and secondary breast carcinoma specimens from patients classified into 3 groups: (1) both specimens excised on the same occasion (61 patients); (2) specimens obtained on separate occasions with no intervening treatment (43 patients); (3) specimens obtained on separate occasions with intervening chemotherapy and/or irradiation (25 patients). There were highly significant linear correlations (P less than 0.001) between the concentrations of ER (expressed as log10) in primary and secondary specimens in all groups. The relationship between PgR concentrations in primary and secondary specimens in groups 1 and 2 was highly significant, although there appeared to be a greater tendency for loss of PgR in sequential, than in simultaneous secondary biopsies. When expressed in terms of hormone receptor status (HRS), the same rate of discordance was observed in groups 1 and 2 (30% when concentrations were expressed in terms of cytosol protein). In group 1 the major cause of discordance was the occurrence of receptor +ve secondaries in association with receptor -ve primaries, possibly because of the high cellularity of many involved axillary nodes. In group 2, the major cause of discordance was the occurrence of receptor -ve secondaries derived from receptor +ve primaries. In both groups discordance in PgR status was more frequent than in ER status. In group 3, overall discordance in HRS was 24% and was due equally to ER and PgR; however, the high concordance rate for PgR was probably due to the fact that the tumours were initially PgR -ve, and the secondaries were also -ve. These results confirm that ER content tends to be stable, even after long periods of time and the administration of chemotherapy and/or irradiation. Progesterone receptor content is much less stable, and may decrease during quite short time intervals even in the absence of treatment.