Retinoids have been shown to modulate cell growth and differentiation in a variety of human tumor cell types, but their effects on B-cell non-Hodgkin's lymphomas (NHL-B) have not been explored. In this study, all-trans retinoic acid (ATRA) in the free form and liposome-encapsulated form (L-ATRA) were used to determine effects on fresh NHL-B patient cells as well as cell lines recently established from both HIV-negative and -positive NHL-B patient biopsies. Both ATRA and L-ATRA were found to inhibit cell proliferation in NHL-B cells. However, L-ATRA was found to be superior to free ATRA in inhibiting cell proliferation of NHL-B cells and resulted in greater than 90% cell growth inhibition in a dose-dependent manner. In addition, L-ATRA also induced high levels of apoptosis in NHL-B cells in vitro. To delineate the apoptotic pathways involved, the expression of the apoptosis suppressor oncogene bcl-2 was evaluated in different NHL-B cells with and without the t(14;18) chromosomal translocation. After L-ATRA exposure, more than a 50% reduction in the expression of bcl-2 protein was observed. bcl-2 message levels were also down-regulated in the L-ATRA-sensitive NHL-B cells. Bax protein levels were analyzed and found to be up-regulated in L-ATRA-sensitive NHL-B cells. Similar results were observed in sensitive AIDS/lymphoma cell lines. Experiments using an RAR-alpha antagonist (RO 41-5253) showed that both the proliferation inhibition and apoptosis induced by L-ATRA could be blocked in NHL-B cells. The findings of the present study indicate that L-ATRA may possess therapeutic potential in blocking cell proliferation, inducing apoptosis, and