A few years ago one of the authors in unpublished results showed that rhodamine-B would precipitate pepsin from solution. Since the well known proteolytic ferment precipitating agent, safranine, also precipitates insulin,1 it was decided to determine if rhodamine-B would precipitate insulin and if so, whether the dye could be removed without inactivation of the insulin. Our experimental results show that insulin is readily precipitated by rhodamine-B at pH 7.2. The complex is soluble in distinctly acid solution, probably with decomposition into its components, for the dye can be practically completely removed by extraction with isoamyl alcohol, leaving the insulin behind in the aqueous layer.To 1 cc of insulin, Lilly, (40 units per cc) 1 cc of a phosphate buffer solution of pH 7.2 was added, followed by 1 cc of a saturated solution of technical rhodamine-B which was either centrifuged or allowed to stand for several days to remove water insoluble impurities. Almost immediately after addition of the dye a ...