Seven Market Administrators' laboratories, with two years of experience in collaborative testing of dairy products, analyzed 10 samples of milk in quadruplicate. Eight of the samples were fresh, raw milk taken from farm bulk tanks and represented four breeds of dairy cattle; two samples were pasteurized skim milk. All samples were preserved with HgCl2. The protein content was measured by the Udy dye binding procedure. Analysis of variance showed a significant betweenlaboratory variation. The within-laboratory variation was slight. Laboratory means were between 98.56 and 101.53% of the overall mean for each sample. Between-laboratory coefficients of variation were 0.52—1.13%. Thus the dye binding method is highly reproducible within laboratories; between-laboratory reproducibility is acceptable