Platelet functions were studied in 16 patients with multiple myeloma (MM) and 4 with primary macroglobulinemia (PM). The fall in immunoglobulins on therapy was correlated with the status of platelet function. Effect of incubation of normal platelets with immunoglobulins derived from these patients on kaolin-induced platelet factor 3 (PF3) release was studied to elucidate the mechanism of platelet function defect. Results show increased bleeding time, absence or poor platelet adhesion and aggregation, poor PF3 availability and reduced total PF3 in platelets more consistently in PM but to a lesser extent in MM. Normalisation or partial reduction in globulins on therapy was associated with improvement in platelet functions. Incubation of normal platelets with immunoglobulins enhanced the kaolin-induced PF3 availability. It is suggested that in vivo platelet activation may bring about the PF3 release which subsequently manifests as poor PF3 availability and reduction in its total contents.