In in vitro assay the hemolytic activity of live isolates and clones of Trichomonas gallinae was investigated. The fresh isolates and clones were tested against human erythrocytes of groups A, B, AB, and O, and against erythrocytes of six adult animals of different species (rabbit, rat, chicken, horse, bovine, and sheep). Results showed that each of the isolates and clones tested lysed all human blood groups, as well as rabbit, rat, chicken, horse, bovine and sheep erythrocytes. No hemolysin released by the parasites could be identified. Parasite culture supernatants of hemolysis assays, tested in the presence of human erythrocytes did not induce any hemolytic activity. Trichomonads culture supernatants from 18, 24, and 48 hr extracts of sonicated parasites or cold killed organisms tested with human erythrocytes did not exhibit hemolysis. Although an hemolytic activity of T. gallinae was clearly demonstrated, a relationship between hemolysis and cell pathogenicity is not yet clearly established.