Internal contamination by 131I among the personnel in a nuclear medicine laboratory was surveyed from January to July, 1979. Because of the formulation change made by the manufacturer in April to reduce volatility, it was possible to study two different therapeutic solutions of 131I. Internal contamination of personnel who administered 131I was found to be 90 times less after the reformulation. Compliance with a recent NRC regulatory guide of less than 40 nCi (1.48 kB1) of 131I thyroid burden as a quarterly average should not be difficult using the new, less volatile 131I; however, strict precautionary measures are essential to minimize contamination, which is common in the nuclear medicine environment.