Three hybrids derived from CD5+ B cell chronic lymphocytic leukemia (B-CLL) and their parental B cells were studied for phenotypic evolution, immunoglobulin (Ig), tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) secretion. When phenotypic evolution was examined, hybrids showed the loss of classical B cell markers, indicating that they follow the same pattern of phenotypic differentiation as normal B cells. Hybrids displayed spontaneous high Ig secretion, which did not appear to be modified through stimulation by phorbol 12-myristate 13-acetate (PMA), recombinant interferon-gamma (rIFN-gamma) and Staphylococcus aureus Cowan I (SAC). Parental cells secreted minimal amounts of Ig spontaneously or through IFN-gamma and SAC stimulation, whereas PMA succeeded in increasing this secretion. An opposite pattern was observed when TNF-alpha and IL-6 secretion an expression at the mRNA level were assessed in hybrids and parental cells. TNF-alpha and IL-6 were spontaneously secreted by parental cells and this secretion was increased after PMA and SAC stimulation, both cytokine secretion and expression at the mRNA level were negative in hybrid cells. The absence of expression of these cytokines could be explained either by chromosomal loss or by down regulation. These results indicate that when parental CLL cells are induced to differentiate in the heterohybrid model, they acquire high spontaneous secretion of Ig, lose the classical B cell phenotypic markers and down regulate the expression of the cytokines studied.