The response to interferon-alpha (IFNalpha) treatment in leukemias of the B-cell lineage shows a marked heterogeneity. A distinct subset of patients with B-CLL responds to treatment with IFNalpha, 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 IFNalpha 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 IFNalpha in CLL. In human lymphoid cell lines, IFNalpha 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 IFNalpha seemed to have both stimulatory and inhibitory effects on proliferation, but it consistently stimulated tyrosine phosphorylation. Moreover, the clinical response of B-CLL to IFNalpha did not correlate with the activation of tyrosine kinases nor with the inhibition of cell growth in vitro. Therefore, the assessment of IFNalpha-induced tyrosine phosphorylation of cytosolic phospho-proteins does not allow to predict the treatment response to IFNalpha in CLL patients.