Low doses of Nutlin-3 enhance the effects of a low dose of Peg IFN-α 2a on the apoptosis of PV hematopoietic progenitor cells. Flow cytometric analysis of normal and PV CD34⁺ cells after 4 days of treatment with 200nM of Nutlin-3 alone or 200 U/mL of Peg IFN-α 2a alone or in combination. None of the treatments increased the number of apoptotic cells in cultures of normal CD34⁺ cells (n = 4). By contrast, Peg IFN-α 2a and Nutlin-3 alone increased to a limited degree apoptosis of PV CD34⁺ cells, whereas Nutlin-3 combined with Peg IFN-α 2a induced apoptosis in PV CD34⁺ cells to a far greater extent. *P < .05. **P < .01. n = 4.