Internal tandem duplications (ITD) and D835 point mutations of the receptor tyrosine kinase (RTK) FLT3 are found in a high proportion of cases with acute myeloid leukemia (AML). These genetic aberrations may lead to the constitutive activation of the receptor, thus providing the molecular basis for a persisting growth stimulus. We have screened 69 AML-derived cell lines for FLT3 mutations. Four of these cell lines showed ITD of the FLT3 gene, none carried a D835 point mutation. Two cell lines (MUTZ-11 and MV4-11) expressed exclusively the mutated allele, the other two cell lines (MOLM-13 and PL-21) displayed a mutated and the wild-type version of the gene. Although mutationally activated FLT3 is supposed to substitute for the stimulatory signal of a growth factor, one of these cell lines (MUTZ-11) was strictly cytokine-dependent. FLT3 transcripts were found in all four cell lines, but the constitutively phosphorylated receptor protein was clearly detectable only in cell line MV4-11, possibly explaining why MUTZ-11 cells were growth-factor dependent. Thus, not all FLT3 ITD-positive cells express high levels of the active receptor protein, a finding that might be of relevance for a possible future application of a kinase inhibitor as therapeutic agent. It had been described that STAT-5 phosphorylation was part of the FLT3 signalling chain and that STAT-5 molecules were constitutively phosphorylated in FLT3 ITD-positive cells. Although we observed the constitutive phosphorylation of STAT-5 molecules in FLT3-mutant cells, FLT3 ligand (FL) did not induce STAT-5 phosphorylation in FLT3 wild-type cells. These results suggest that the signalling mechanisms of the mutated FL receptor differ at least to some extent from those conferred by wild-type FLT3. In conclusion, (1) not all cells with FLT3 ITD express significant amounts of the mutated receptor protein; (2) signals downstream from wild-type and mutant FLT3 receptors are not 100% identical; and (3) MV4-11 represents a model cell line for FLT3 ITD signalling.