AIMS: To evaluate the feasibility of an interlaboratory quality control programme in immunohistochemistry. METHODS: Several pathology laboratories were asked to carry out immunohistochemical oestrogen receptor staining on a set of freeze dried cryostat sections of breast cancer tissue. The sections and protocols for staining and semi-quantitative scoring were mailed to the participating laboratories in two trials. The oestrogen receptor content of the breast cancer samples was determined by radioligand binding assay on the tumour cytosol. RESULTS: In the first trial 11 laboratories (response rate 60%) participated. Eight (73%) of the participants scored within a 95% confidence interval and all but one correctly classified the tumour as receptor positive. In the second trial all 20 participating laboratories (response rate 55%) correctly scored one tumour sample as negative and 18 of them (90% of respondents) correctly classified the two other tumour samples as receptor positive. In a quantitative evaluation a histochemical score within 95% confidence interval limits was provided by eight (40%) and 12 (60%) of the participants. CONCLUSIONS: Semiquantitative scoring of immunocytochemical staining is valuable for performing correlative inter-laboratory studies, although this scoring protocol may not be required for diagnosis or prognosis. Significant inter-laboratory variability exists, leading to qualitatively correct receptor classification in 100% of receptor negative and 80% of receptor positive cases, and quantitative agreement in only about half of the cases. The perceived variability is not caused by systematic differences in the choice of the immunocytochemical technique, or the mailing of freeze dried sections. Quality control programmes should be included in the standard procedures of each diagnostic immunohistochemistry laboratory.