1. Urinary cyclic nucleotide excretion was studied in 51 patients with malignant tumours and in 24 control subjects.
2. Adenosine 3′:5′-cyclic monophosphate excretion was normal in patients with tumours.
3. Urinary excretion of guanosine 3′:5′-cyclic monophosphate was significantly greater than normal in patients with lymphoid tumours (both sexes) and in male patients with myeloid tumours or malignant melanoma.
4. Guanosine 3′:5′-cyclic monophosphate excretion was normal in patients with breast carcinoma or colonic adenocarcinoma.