AbstractExperiments were made to identify the sources of error in the Rao procedure for available lysine in cottonseed and peanut meals and to estimate the magnitude of each. The partition of squares in analyses of variance of the data reported revealed that the major error can be attributed to sampling. The variances due to other possible sources of error, such as dinitrophenylation, hydrolysis, chromatography, etc., were not significantly different from the error variance, which (after the sampling error is accounted for) corresponds to confidence limits at the 5% level of probability of ±0.5% of the mean of the available lysine concentration.