In this article we report our initial clinical experiences in connection with immunomagnetic isolated CD34-positive cells from peripheral blood progenitor cells. Six patients, five with breast cancer and one with non-Hodgkin's lymphoma, were mobilized by chemotherapy and G-CSF (5ug/kg). CD34-positive cells were isolated by means of immunomagnetic beads (Dynalbeads) and Isolex 300 Cell Separator (Baxter, USA). Mean purity of isolated CD34-positive cells was 97% (94.7-99.7) and mean yield was 54% (35-68). Three patients were treated with high dose therapy followed by reinfusion of CD34-positive cells as stem cell support. Recovery of neutrophils (> 0.5 x 10(9) leucocytes/liter) occurred at day 8, 11 and 13 and of platelets (> 20 x 10(9) platelets/litre) at day 9,14 and 32. It is concluded that immunomagnetic isolated CD34-positive cells give high purity and yield. Although use of CD34-positive cells reduces the content of contaminating tumour cells in the graft, breast cancer cells were still detectable in two out of five CD34-positive cell products.