The available methionine and cysteine of proteins were determined by chemical methods after preliminary enzymic hydrolysis. 2. The values for the available methionine and cysteine contents of pure proteins (casein and bovine serum albumin) estimated by chemical methods were similar to those for the total content determined by the method of Moore, Spackman & Stein (1958). 3. Reductions of 15 and 11% respectively, when compared with unprocessed samples, were found in the available methionine contents of sweetened and unsweetened, condensed milks; of roller-dried milk and whey powders, and of mackerel sterilized at 126 degrees, the reductions were 22, 14 and 19% respectively. 4. The available cysteine content of sweetened, condensed milk was reduced by about 32%, whereas for mackerel sterilized at 115 and 126 degrees it was reduced by 64 and 75% respectively. 5. The contents of total sulphur amino acids for these food products did not differ from those for the unprocessed samples. 6. Values obtained for available S amino acid contents by rat bioassay confirmed the results of the in vitro estimations.
