Tumor cell uptake of tritiated thymidine, expressed as the labeling index (LI), was determined prior to treatment of 25 patients with disseminated breast carcinoma. All patients subsequently received combination chemotherapy with adriamycin, cyclophosphamide, and an antimetabolite (usually 5-fluorouracil), with or without vincristine. The LI was significantly higher in responders to chemotherapy than in nonresponders (mean, 15 vs. 7.1; P<0.01). Other pretreatment variables examined did not show a significant association with likelihood of response. Measurements of the tumor cell LI in patients with accessible tumor may be of benefit in selection of treatment; 0 of 9 patients with an LI less than 9 had a response in our series in contrast to 11 of 16 patients with an LI greater than 9 who had a response (P= 0.001).