Attempts were made to isolate the substance or substances in whey responsible for the inhibition of infection with the barley stripe mosaic virus (BSMV). The fractions isolated were bioassayed to determine their potency. The bioassay method is described. Paper electrophoresis was used to indicate the composition of the fractions.Potent proteinaceous substances were obtained by dialysis, selective denaturation with ethanol, and fractional precipitation with ammonium sulphate. A progressive increase in potency was associated with an increase in the proportion of material that moved towards the cathode in electrophoresis at pH 8.6.Elutions from the electrophoretic bands gave evidence of very high potency in the two substances that moved towards the cathode but there was also evidence of degradation during elution. Quantitative bioassays of eluted materials were uncertain because of indications that the eluant had dissolved something from the paper.Development of electropherograms with Solway purple had no derogatory effect on the inhibitory material but ninhydrin destroyed all activity.