One batch of lyophilized calf uterine cytosol was analyzed for estrogen and progesterone receptor (ER and PgR, respectively) content by 12 members of the EORTC Receptor Group on three different occasions over a total study period of 1 yr: One vial was included with each of 20 consecutive batches of routine tumor analyses between December 1983 and May 1984. Two vials were simultaneously assayed between July and August 1984 (within-run variation). One vial was analyzed at the end of 1984 (November-December). The overall mean ER and PgR values did not change systematically over the total study period of 1 yr. Within the various laboratories, the between-run variations of both ER and PgR assays were considerable and differed from one institution to another (7-26%). For both ER and PgR measurements the average within-run (n = 2) and between-run (n = 20) coefficients of variation were similar (8-9% and 16-17%, respectively). Comparison of the results from multiple sequential assays (c.v. = 12.9%) with those from single assays (c.v. = 21.2%) showed that about 60% of the between-laboratory variance in ER could be explained on the basis of the between-run variance. With regard to PgR analysis, however, the between-laboratory variance decreased only 25%. Standardized use of one type of protein assay (Coomassie brilliant blue) and a standard protein solution (human serum albumin) has decreased the between-laboratory variation of the protein analysis results to less than 15%.