Rosette formation between human lymphocytes and horse red blood cells could be promoted by a low pH medium, overnight incubation and a temperature of 4°C. The percent of sheep, horse and human rosette-forming cells in the peripheral blood were 71.7 ± 1.8, 30.5 ± 2.8 and 28.3 ± 3.4 respectively. However, their percentages in thymuses were 97.1 ± 1.1, 91.4 ± 2.4 and 89.0 ± 3.4. Using preparations of isolated subpopulations, it was observed that the horse and human red cell rosette-forming cells were probably also “early” sheep red cell rosette-forming cells. Rosette formation with all three types of red blood cells were inhibited by a preparation of Fetuin-glycopeptide.