A number of experimental observations suggest that the proto-oncogene c-abl participates in the regulation of hematopoietic cell growth. We used an antisense strategy to study the relationship between c-abl expression and hematopoietic cell proliferation and differentiation. Purified normal human bone marrow-derived CD34+ cells were obtained by immunomagnetic selection and incubated with 18-base-unmodified antisense oligodeoxynucleotides complementary to the first six codons of the two alternative first exons of c-abl, la and lb. At the end of incubation, an aliquot of cells was assayed for clonogenic growth and the remainder was used for flow cytometric analyses. Cell kinetics were evaluated by means of both single parameter DNA and bivariate DNA/bromodeoxyuridine (BrdU) flow cytometry. Apoptosis was routinely studied by DNA flow cytometric analysis and, in some cases, also through DNA agarose gel electrophoresis for detection of oligonucleosomal DNA fragments. Expression of differentiation markers was studied by flow cytometry. Exposure to antisense oligonucleotides specifically inhibited the accumulation of c-abl mRNA in CD34+ cells. Preincubation with the c-abl antisense oligomers reduced the proportion of cells in S-phase from 19% +/- 5% (mean +/- SD) to 7% +/- 4% (P < .05), and BrdU labeling from 13% +/- 6% to 6% +/- 3% (P < .05). Flow cytometry and DNA agarose gel electrophoresis showed that treated CD34+ cells accumulated in the G0/G1 region of the DNA histogram with no evidence of either differentiation or apoptosis. By contrast, both growth factor deprivation and exposure of CD34+ cells to the tyrosine kinase inhibitor tyrphostin AG82 clearly induced apoptosis. When cells were preincubated with antisense oligonucleotides and then plated for evaluation of colony formation, this resulted in a significant inhibition of colony forming unit granulocyte-macrophage growth (from 44 +/- 15 to 22 +/- 9; P < .01) but had no effect on burst-forming unit erythroid growth (24 +/- 11 v 21 +/- 11; P < .05). These results suggest that c-abl expression is critical for entry of human CD34+ hematopoietic cells into S-phase and for their differentiation to granulocyte-macrophage progenitors. They also indicate that other tyrosine kinases besides p145c-alb are active in the prevention of apoptosis, so that inhibition of c-abl RNA accumulation arrests CD34+ cells in G0/G1 without activating programmed death.