The formation of heteroag-glutinins against human group O, horse and sheep erythrocytes was studied in groups of chickens injected with various materials. Production of antibodies cross-reacting with some or all of the test cell types was induced both with chicken thyroid extract, with or without adjuvant, and with inactivated infectious bronchitis vaccine with adjuvant. The chicken tissue in the injection materials is considered to be responsible, to a great extent, for the stimulations recorded. Also complete Freund’s adjuvant alone, however, had a stimulating effect. The heterogeneity of the antibody response as regards specificity, and differences in the responsiveness of individual chickens, were demonstrated.