We examined peripheral blood progenitor cell (PBPC) collections and CD(34+)-selected fractions cultured in PIXY321, a fusion protein comprising analog interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) domains, for the presence of contaminating tumor cells from 14 patients with advanced-stage breast cancer. Five of the 14 (36%) pre-culture PBPC specimens contained immunocyto-chemically (ICC)-detectable tumor cells using two different cocktails of monoclonal antibodies (mAbs). After 10 days in culture with PIXY321, the CD(34+)-selected fractions showed a median 23.6-fold expansion of hematopoietic cells. No ICC-positive tumor cells were detected in any post-culture specimens. We conclude that in vitro expansion of CD(34+)-selected PBPCs with PIXY321 can expand hematopoietic cell populations apparently without risk of expanding contaminating breast cancer cell populations.