Shinjefd, Reading RG2 @TTannins are widely present in higher plants, and their presence in some food grains may significantly impair the nutritional quality. Thus for example, in coloured-flowered varieties of field beans (Vicia fuba L.) the seeds are rich in condensed tannins and their inclusion in the diet of single-stomached animals has been reported to depress the apparent digestibility of the protein (cf. MartinTanguy et af. 1977; Ronnenkamp, 1977). Similarly in sorghum grain (Sorghum vulgare Pers.) the protein quality in some genotypes is severely limited by the high level of tannins in the testa, and different varieties exhibit wide differences in digestibility that reflect corresponding differences in tannin content.The presence of tannins complicates the interpretation of amino acid analysis and other compositional information in terms of nutritional quality. We need to take the digestibility of the amino acids into account in our tests, and there are several microbiological assay methods that are very promising for this purpose and are potentially of great value to the plant breeder in guiding the selection of improved food grains. Compared with animal feeding tests they are quick and precise, and much more economical of test materials. Of these methods, the Streptococcus zymogenes assays for available methionine and for relative nutritional value (RNV) have been extensively tested with a variety of high-protein feedstuffs and with rice ( 0 7 y Z U sutiva L.), sorghum, barley and field beans (Ford & Hewitt, 1979a,b,c). Table I shows the results of amino acid digestibility tests with chicks on ten sorghum varieties of graded tannin content, and for comparison the results of microbiological tests and values for dye-binding lysine. The chick assay values for 'average amino acid digestibility' were negatively correlated (7-0.82 ; P < o . o I ) with tannin content, as were the microbiological assay values for available methionine (~-0 . 9 5 ; P < O . O O I ) and for RNV (~-0 . 9 7 ; P