Two Gramineae species, oat and maize, are compared with wheat and barley to see if they contain lectins which are structurally and functionally similar to the Hordeae lectins. Four distinct criteria were examined: localisation of lectin activity in the seed, ability to agglutinate a defined type of erythrocyte in a reaction reversed by monomers or oligomers of N-acetyl-D-glucosamine, ability to bind to the affinity matrix p-aminobenzyl-1-thio-2-acetamido-2-deoxy-β-D-glucopyranoside-substituted Sepharose, and cross-reactivity with monospecific antisera raised to wheat-germ agglutinin. Results indicate that the very close relationship found between the lectins of wheat, barley and rye cannot be extended to those species of Gramineae outside the tribe Hordeae.