The invention provides a non-enzymatic method for the release of cells which have been positively selected from a heterogeneous cell suspension by antibody-mediated binding to beads or other solid support. The method entails forming within the cell suspension a complex comprising the solid support linked to a primary antibody, which in turn is specifically bound to a cell surface antigen on the target cells. The complex is separated from the cell suspension, and then contacted with a specific peptide which binds to the primary antibody, displacing the antibody from the cell surface antigen, thereby releasing the target cell from the complex. The invention also provides methods for positive/negative cell selection wherein target cells having a first antigen are selected from a heterogeneous cell suspension containing undesired cells having a second antigen. The invention also provides methods for identifying a specific peptide useful for the release of a target cell from the binding of a specific primary antibody. The methods of the invention are particularly useful for the positive selection of CD34+ hematopoietic stem cells and the concomitant purging of undesired tumor cells or lymphocytes from the positively selected cell population. The purified CD34+ cell composition is then useful for reinfusion to a cancer patient after high-dose therapy in order to reconstitute the patient's immune system.