Guar gum, powdered endosperm from the seeds of the legume Cyamopsis tetragonolobus, is a galactomannan which contains 2.5-4.5% of a proteinaceous component. Data presented for 11 bulk commercial samples show that the most abundant amino acids are glycine, glutamic acid, aspartic acid, serine and alanine, but that their relative proportions vary considerably. The proportions of other amino acids, e.g. histidine, isoleucine, phenylalanine, threonine, tyrosine and valine are remarkably constant. There appear to be some correlations between viscosity and the glycine, arginine and proline contents. Data for the amino acid compositions of several chemical derivatives of a medium-viscosity gum guar sample indicate that the proteinaceous component is retained but suffers variable modification. The amino acid profile for guar gum differs extensively from those for gum arabic and gum karaya; this provides a useful analytical supplement to sugar determinations for studies of admixtures of these hydrocolloids.