Cell lysates from 16 strains of eight Leishmania species were used to test haemagglutination activity (HA) against a variety of RBC. HA was detected using native or neuraminidase-treated rabbit RBC; it was found in promastigotes of all the Leishmania strains tested and in axenic amastigotes of L. mexicana. The HA was trypsin-sensitive, heat-resistant and partially dependent on divalent cations. The HA was inhibited by amino-sugars, LPS from E. coli K 235, fetuin and heparin. The HA is probably located on the surface of promastigotes, as shown by the same sugar-binding specificity when live cells were used in inhibition tests. Leishmania promastigotes were agglutinated with neoglycoproteins NAc-glc-BSA and NAc-gal-BSA. This agglutination was blocked by galactosamine, glucosamine and sialic acid, but not by glcNAc or galNAc. The level of HA is increased in axenic amastigotes when compared to promastigotes. In general, HA was found at a higher titre in infective compared to uninfective strains of Leishmania. These results suggest that the haemagglutinin could play a role in the vertebrate phase of the parasite life cycle, possibly in macrophage attachment or invasion.