A protein isolated and purified from extracts of bark strips from mature rubber (Hevea brasilinsis) agglutinated erythrocytes from rabbits and all human blood types except AB. No agglutination was detected with erythrocytes from sheep, rates or mice. Proteolytic treatment of Hevea bark lectin (HBL) abolished haemagglutinin activity. The M(r) determined by SDS-PAGE was 40 k and that estimated from gel filtration was 140 k. Fetuin, asialofetuin, bovine submaxillary mucin and asialosubmaxillary mucin inhibited HBL-induced agglutination of rabbit erythrocytes. The HBL showed maximum haemagglutination activity over the pH range 4.5-9.5 and heat stability up to 60 degrees.