Expression profiles were generated for the haemopoietic tyrosine kinase receptors (HGF-TKRs or class III TKRs) by PCR on cDNA samples (RT-PCR) using a degenerate primer set. Each profile consisted of primary and secondary, i.e. enriched for less-expressed sequences, fingerprints. This method was applied on FACS-purified haemopoietic CD34+ cells, both from bone marrow (BM) and umbilical cord blood (UCB), and on mature cells from peripheral blood. CD34+ BM cells showed expression of c-fms. flt3, whereas CD34+ UCB cells expressed c-fms and, to a lesser extent, c-kit and flt3. In mature blood cells, only c-fms was observed in monocytes and a weaker flt3 expression in monocytes and T lymphocytes, whereas no known class III TKRs were detected in B lymphocytes and polymorphonuclear cells (PMNs). In all fractions a novel band could be observed, which appeared to be RET. Expression of RET was confirmed by RT-PCR and showed the highest levels in monocytes, followed by PMNs and CD34+ cells. B lymphocytes revealed low levels of expression. RET is known to be essential in neural development. Our results suggest a possible role for this receptor in haemopoiesis.