1. The activity of diamine oxidase (EC 1.4.3.6) in normal rat kidney cells and in normal rat kidney cells transformed by avian sarcoma virus (B77 strain) growing in tissue culture varies with the stage of growth. There is an initial stimulation of activity by 24h after seeding, followed by a steep decline during exponential growth (48-72h). Enzyme activity decreases even further as the cells reach saturation density (confluence) after 4 days in culture when the activity in normal rat kidney cells is twice as high as that in transformed cells. 2. Differences of about the same order of magnitude are observed between transformed human cells HeLa, HEp2 (a human epithelioid carcinoma) and normal human fibroblasts, in chicken cells between normal myeloblasts and leukaemic myeloblasts, and in rats between biopsy material from normal mammary tissue and 9,10-dimethylbenz[a]anthracene-induced mammary tumours. 3. Polyamine oxidase activity also varies with the growth of transformed rat kidney cells, but shows no significant variation with the growth of normal rat kidney cells between 24 and 96h after seeding. The activity in cells at confluence is from 3- to 5-fold lower in the transformed than in the normal rat kidney cells. 4. A similar 5-10-fold decrease in activity has been found in 9,10-dimethylbenz[a]anthracene-induced mammary tumours in rats and in human oesophageal tumours. 5. Possible reasons for these observations and the contribution of these two enzymes to cellular putrescine concentrations are discussed.