Abstract The ultrastructure and 3H-thymidine uptake of rosette-forming cells (RFC) for chicken erythrocytes have been studied in normal mice, in mice immunized (primed) 110–180 days previously, and in mice undergoing primary or secondary immunization. Ultrastructurally most RFC were lymphocytes, though plasma cells, macrophages, and polymorphonuclear (neutrophilic) leukocytes also formed rosettes, especially in recently immunized mice. No consistent qualitative morphological differences among the lymphocyte rosette-formers of the different groups were found. Quantitative differences with respect to the percentages of inactive, intermediate, and activated lymphocyte rosette-formers were found, however, between the cells of recently immunized mice and those of mice not recently immunized. A similar difference was found in the percentage of RFC incorporating 3H-thymidine in the two groups. Neither the ultrastructural nor the radioautographic studies revealed significant differences between RFC of normal mice and those of primed mice, nor between RFC of mice undergoing a primary response and those of mice undergoing a secondary response.