Understanding the mechanisms inherent to malignant cell eradication is a major determinant for cancer therapy. Recent data have demonstrated that apoptosis may be one of the mechanisms through which both cytotoxic and differentiating drugs may eliminate malignant cells. Treatment of acute promyelocytic leukemia (APL) by all-trans retinoic acid (ATRA) is the first model of differentiation therapy allowing achievement of more than 90% complete remission (CR). However, disease-free survival (DFS) is short if patients are not subsequently treated with chemotherapy. In order to address the question of APL cells' elimination during ATRA therapy, we studied phenotypic and molecular features of 14 APL cases relative to cell survival in primary culture in the presence or absence of ATRA. Compared to other acute myeloid leukemia (AML) subtypes, APL cells in short-term suspension culture present a better survival rate (P < 0.001). After incubation with ATRA, cell survival was not altered and was correlated with a concomitant absence of apoptosis, despite a significant decrease of the BcL-2 protein in APL differentiated cells. Indeed, after 6 days of culture, only 3 +/- 0.5% of APL cells exhibit morphological features of apoptosis after ATRA treatment compared to 30 +/- 5% in HL-60-treated cells. Treatment of APL cells with 9-cis RA, 13-cis RA or analogs of RAR alpha or RXR alpha also failed to induce apoptosis. Treatment of either APL or ATRA-differentiated APL cells with 40 microM etoposide resulted in DNA fragmentation and morphological changes characteristic of apoptosis in 23 +/- 5% cells after only 20 h of treatment and 68 +2- 3% after 48 h suggesting that other pathways of apoptosis are still functional in APL cells. Though these in vitro data cannot fully represent the mechanism of cell death and cell elimination in vivo, they clearly indicate that ATRA alone may not induce leukemic clone eradication by apoptosis correlating with the persistence of minimal residual disease and constant relapse after CR obtained with ATRA alone.