Abstract Lupinus albus seeds (20 samples from 10 different lines or cultivars) with protein contents distributed from 23.8 to 48.4% were analysed for their amino acid composition with great accuracy (from 6 hydrolysates per sample). Amino acid levels in seeds increased as linear functions of nitrogen content with correlation coefficients close to unit in all the lupin genotypes and phenotypes. Hence the composition of any lupin seed sample can be predicted from its nitrogen content. Amino acid in total seed protein varied as hyperbolic functions of nitrogen content. The same was true for nitrogen-to-protein conversion factors. By controst, the composition of storage proteins accumulated in seeds remained constant and independent of seed protein content.