Summary.-Adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) levels of peripheral blood mononuclear cells were measured in 34 patients with various types of solid tumours. The mean ADA activity was found to be significantly lower than in controls (P < 0.005). Patients with nonresectable tumour or with recurrence after radical surgery showed low ADA levels, while patients operated upon and without recurrence had enzymatic activity not different from that of normal controls. The mean value of PNP activity was similar to that of normal controls; no differences were observed between operated patients without recurrence and cases with nonresectable tumour or with recurrence after surgical treatment. No effects on ADA and PNP levels appeared to be induced by chemotherapy.