The response to interferon-alpha (IFNα) treatment in leukemias of the B-cell lineage shows a marked heterogeneity. A distinct subset of patients with B-CLL responds to treatment with IFNα, while the drug has no therapeutic effect in the majority of patients. The mechanism of this phenomenon is poorly understood. The cellular events induced by this cytokine mediated by a number of specific signaling events. Therefore, we studied the effect of recombinant IFNα on tyrosine phosphorylation and proliferation of cytosolic proteins in human cell lines and in freshly isolated B-CLL cells in order to test the potential value of these events as a pretreatment test for IFNα in CLL. In human lymphoid cell lines, IFNα induced tyrosine phosphorylation of multiple cytosolic proteins in a time- and concentration-dependent manner. This effect correlated with its growth-inhibitory effect in almost all cell lines. In marked contrast, in freshly isolated B-CLL cells IFNα seemed to have both stimulatory and inhibitory effects on proliferation, but it consistently stimulated tyrosine phosphorylation. Moreover, the clinical response of B-CLL to IFNα did not correlate with the activation of tyrosine kinases nor with the inhibition of cell growth in vitro. Therefore, the assessment of IFNα-induced tyrosine phosphorylation of cytosolic phosphoproteins does not allow to predict the treatment response to IFNα in CLL patients.