Two kinds of humoral factors were observed in 2 orders, 7 families, 10 genera, and 15 different species of Korean ascidians. They are the naturally occuring hemagglutinins and/or hemolysins against human erythrocytes A, B, and 0. All but two species showed aggregative activity, although there were considerable variations in titer. The weak agglutinating and lytic activities were increased in the presence of $Ca^{++}$. Much higher activities of agglutination and/or lysis were shown in the hemolymph than extracts from tissues, and a higher response was shown in adults than in juveniles. No distinct differences from collected locations were observed. The hemolymph of Ciona intestinalis showed a strong hemolytic (cytotoxic) and weak agglutinins capacities. In addition, hemolymph of Styela plicata and Styela clava clava also showed hemoagglutining and hemolytic activities. Botryllus tuberatus had hemagglutining and weak lytic activities. Other species showed only hemagglutining activity. These agglutining activities are probably responsible for carbohydrate recognition in solitary or colonial ascidian. The lytic activity is probably responsible for antibacterial defense and nonfusion reactions between allogeneic colonial ascidians, especially the genus of Botryllus. The occurrences of humoral factors in ascidians were independent of their geographic distributions.