Abstract We determined the position of the breakpoint within the bcr gene in 22 patients with Philadelphia-positive chronic myelocytic leukemia using conventional Southern-blots and analyzed its relationship to thrombopoiesis. After DNA digestion with restriction endonucleases (Hind III, Bam HI and Bgl II), we localized the breakpoint in bcr using two genomic probes. The location of the breakpoint within the bcr was assigned to one of five zones. Breakpoints in zones 1 and 2 were grouped as “5′”, and those in zones 3, 4 and 5 as “3′”. Thus we subdivided patients with bcr rearrangements into those with genomic breaks at either 5′ or 3′ of the Bam HI site, just upstream of exon 3. Nine patients had 5′ breakpoints and 13 patients had 3′ breakpoints. The platelet counts of 3′ patients were significantly higher than those of 5′ patients (1395 vs 274 × 10 9/l; p < 0.03). The megakaryocyte counts from bone marrow histological sections in 3′ patients ( n = 12) and 5′ patients ( n = 7) were 63.4/mm 2 and 19.5/mm 2, respectively, with a significant difference of p < 0.006. The mean number of megakaryocyte progenitor cells assayed by in vitro cloning was 128.3 2 × 10 5 bone marrow cells for 3′ patients ( n = 7) compared with 46.3 for 5′ patients ( n = 4). These results suggest that Philadelphia-positive CML patients with 3′ breakpoints have higher thrombopoietic activity than patients with 5′ breakpoints.