The present study has attempted to further delineate the growth factor requirements of peripheral blasts of a patient with CML in acute phase. Phenotypic analysis of leukemic blasts from this patient before culture has shown a homogenous population of CD34+ cells at the onset of blast crisis. In the second and third samples the percentage of CD34+ DR+ blast cells decreased slightly and up to 32% of cells in the third sample expressed the CD19 antigen. Optimal proliferation of cells derived from the first sample required the presence of exogenous sCD23 and to a lesser extent IL7. The stimulatory effects of sCD23 and IL7 were clearly reduced 4 months later and no longer detected after 6 months. This variability in growth factor response along with disease progression may be related to phenotypic differentiation. There was no evidence for lymphoid or myeloid maturation after 4 days of liquid culture. Our results in conjunction with previous studies are in agreement with sCD23-involvement in the complex control of proliferative processes at both normal and leukemic stages, demonstrating that cytokines are critical in determining CML cell proliferation.