The present study presents a simplified method for the measurement of insulin-like activity through its effect on stimulation of glucose-1-(14)C oxidation by adipose tissue. Forty determinations can be carried out by two persons in a single day and only two rats are required. The index of precision is in the neighbourhood of 0.13. When the method was applied to the measurement of total insulin-like activity in dialyzed human serum obtained from fasting subjects, the results obtained varied between 78-1020 μU/ml (465 ± 68 μU/ml). Approximately 40 per cent of the total insulin-like activity was suppressible in the presence of insulin antibodies partially purified from guinea pig serum. This was true both for dialyzed serum from fasting subjects and for acid ethanol extracts of serum. There was no increase in the insulin-like activity of serum upon dilution, provided that the radioactivity of the(14)CO2 formed was directly plotted against the decreasing serum concentrations. A fictitious increase in the insulin-like activity was obtained, however, when the radioactivity was transformed into insulin equivalents by comparison with a standard curve in buffer. It would seem that this is the result of the observation that insulin standard curves in the presence of serum are flatter than is the case in Krebs-Ringer buffer.