The bcl-2 gene encodes a mitochondrial protein that inhibits the onset of apoptosis induced by growth factor withdrawal or cytotoxic agents. Using quantitative flow cytometry and expressing bcl-2 levels as the number of molecules of equivalent soluble fluorochrome (MESF) per cell, we have shown that bcl-2 protein expression in the blast cells from patients with acute myeloblastic leukaemia (AML) is heterogeneous, but not related to FAB type. The blast cells from AML patients with the capacity to grow and survive autonomously in vitro were found to have higher bcl-2 MESF values than those that were dependent upon exogenous growth factors. We have previously reported that the blast cells from 70% of AML patients exhibit autonomous growth and autocrine growth factor production in vitro and that this has been shown to be an important indicator of poor prognosis in AML. High bcl-2 expression has also been associated with a low complete remission rate and poor survival in AML. In the patients whose blast cells exhibited autonomous growth, neutralisation of endogenous GM-CSF resulted in down-regulation of bcl-2 protein, whereas in blast cells from patients whose cells proliferated only in the presence of added growth factors, incorporation of recombinant human (rh) GM-CSF in the culture media resulted in up-regulation of bcl-2. Because CD34 positivity has been reported as another indicator of poor prognosis in AML, we compared bcl-2 expression in cases of CD34 positive AML, CD34 negative AML and CD34 positive normal bone marrow cells. Bcl-2 was found to be strongly expressed on the CD34+ normal bone marrow cells. The blast cells from CD34+ AML patients expressed significantly higher bcl-2 levels than CD34- AML patients. In five cases of CD34+ AML, the bcl-2 levels were determined on purified CD34+ and CD34- blast cell populations. The CD34+ blast cells were found to express significantly higher bcl-2 levels compared with the CD34-blast cells. Our data would suggest that quantification of bcl-2 in AML blast cell may be useful as a prognostic indicator in AML.