Typical assays of bacterial hemolysis mostly involve the use sheep erythrocytes in agar media. To detect novel hemolytic/cytolytic activities of Shigella, we used erythrocytes from a variety of animal hosts( i.e. sheep, horse, human, goat, pig, chicken, bovine, turkey, or goose) with potential differences in lytic sensitivity. Initial studies revealed that many, but not all, Shigella strains lysed horse erythrocytes with occasional straindependent lysis of bovine and/or human erythrocytes. A more detailed analysis was conducted with 112 Shigella isolates, representing all 4 species, using horse, human and sheep erythrocytes. Of these, 41 strains showed no lysis on any media. Of the remaining 71 strains (of all 4 species), 65 showed lysis of any one of the three erythrocyte types, and 6 lysed two or more erythrocyte types. A Listeria control strain always showed hemolysis within 24 hrs incubation at 37°C. However, the Shigella induced hemolysis was only observed after 72 hrs incubation at 37°C and was enhanced by placing the plates at 4°C for an additional 1224 hrs, suggesting heatlability of the Shigella hemolytic activity. Our use of erythrocytes from different sources thus defined unusual hemolytic activities which are both bacterial straindependent and host erythrocytespecific. This approach may be helpful in the detection of novel cytolytic functions of potential relevance to disease pathogenesis in different bacteria.