We investigated the mechanisms of factor-independent growth of four human myeloid leukemia cell lines. The autocrine mechanisms were ruled out by RT-PCR method examining growth factor mRNA. The immunoblotting method showed that many proteins were tyrosine phosphorylated irrespective of the stimulation with growth factors (G-CSF and GM-CSF) in factor-independent cell lines while the phosphorylation was induced stimulation dependently in a factor-dependent cell line. MAP kinase was constitutively phosphorylated in factor-independent cell lines. JAK2 protein was not tyrosine phosphorylated before the stimulation. It was significantly phosphorylated after the stimulation in three factor-independent cell lines although the stimulation did not affect their growth. JAK1 protein was not phosphorylated either before or after the stimulation. In conclusion, constitutive phosphorylation of signaling proteins seemed to be related to factor-independent growth. MAP kinase was involved in the phosphorylation, while JAK1 and JAK2 were not.