Three plasma cell enzymatic reactions--ATPase, acid phosphatase (AP), alpha-naphthyl acetate esterase (alpha-NAE)--were tested in a large number of bone marrow samples from patients with multiple myeloma (MM), benign monoclonal gammapathy (BMG)--idiopathic (iBMG) and secondary (sBMG)--, polyclonal hypergammaglobulinemia (PH), Waldenström's macroglobulinemia (WM) and from normoglobulinemic subjects (NS). The purpose of the present study was to evaluate the significance of these reactions in differential diagnosis of BMG and initial or atypical MM. For each reaction results were expressed as scores and normal limits were statistically established. Our findings in MM, HP and NS are consistent with previous reports (similar enzymatic pattern in NS and PH; depressed ATPase activity, raised AP and alpha-NAE activities in most MM cases). In the BMG class, ATPase activity was normal in about 40% of iBMG, while it was depressed in other cases; most sBMG had diminished ATPase activity. AP patterns were normal for both BMG groups. alpha-NAE activity was normal in 50% of iBMG, while it was raised in the other cases. All the reactions were normal in WM. In a bayesian analysis of cytochemical patterns of NS and MM subjects ATPase showed the highest diagnostic significance, followed by alpha-NAE and AP. ATPase, therefore, seems to be the most useful criterion for the diagnosis of initial MM; low ATPase score BMG patients should therefore be carefully followed-up.