Acid phosphatase (AP) activity of plasma cells was studied in 38 patients with multiple myeloma (MM). The average activity per cell was strong (mean score = 2.42; maximum score = 4) and the percentage of positive cells was greater than 90% in over 71% of patients. The average AP activity per cell was higher prior to treatment (3.06 +/- 0.53) compared to relapse (2.48 +/- 0.77) and remission (1.81 +/- 1.02 (p less than 0.05 and p less than 0.01, respectively). In correlation of AP activity with clinical features at the time of the study, the only significant difference was between kappa and lambda subtype. Patients with lambda MM had a higher average AP activity per cell in remission (2.71 +/- 0.43) as opposed to kappa MM (1.17 +/- 1.06, p less than 0.05 for difference). AP activity was not significantly correlated with degree of bone involvement. However, activity seemed to be a good marker of disease activity.