AbstractFor a series of seven fish, meat and whale meals, there was no correlation between their sulphur content (expressed per 16 g. of N) and their net protein utilisation for rats although the quality of the protein was limited by the sulphur‐containing amino‐acids. The relationship found for a series of human diets cannot therefore be extended to processed material of the type used here. Cystine and methionine accounted for less than two‐thirds of the sulphur found in five of the samples. Digestibility of the sulphur ranged from 53 to 90% and, except for two meals which had the highest ratio of cystine to methionine, was similar to the corresponding figure for nitrogen. The product of sulphur amino‐acid content and sulphur digestibility of the meals correlated with their net protein utilisation for rats. Procedures for estimation of cystine and methionine are discussed.