Cell surface marker analysis was carried out in 50 CLL patients; 15 were preclinical or low count, that is with a total peripheral lymphocyte count well below 15,000/cmm and little or no infiltrative syndrome. Data of the cell surface marker in these 15 patients were compared with those of 15 patients with non-neoplastic benign lymphocytosis. Monoclonal B-cell compartment proliferation was found in low count cases, with analogous immnofunctional characteristics to typical CLL. On the other hand, there was a symmetrical increase in both T and B cell compartments in non-neoplastic lymphocytosis. This suggests that cell marker analysis is a very useful diagnostic tool during the preclinical phase of CLL, as it permits it to be readily differentiated from non-neoplastic lymphocytosis.