The thymus of IQI/Jic mice aged 3 to 15 months was studied chronologically by flow cytometry and immunohistochemistry. Thymic B cells (Thy B) expressing the surface marker B220+IgM+ were detected in both sexes, but were more prominent in females. Thy B appeared as early as 4 months of age in female mice, and the incidence of Thy B-positive mice and average Thy B ratio in thymocytes increased with age, reaching over 90% and 15%, respectively, after 9 months of age. In males, Thy B-positive animals appeared around 9 months of age and its incidence and average ratio increased gradually to 53% and 5%, respectively, at 15 months. The Thy B-rich thymus was thickened with cortical atrophy and medullary hyperplasia. Small-to medium-sized Thy B crowded and often formed follicle-like structures in the medulla. When mice of various ages were injected with mercuric chloride to induce antinucleolar antibody, many Thy B-rich animals could not develop the antibody, implying that these animals are immunologically impaired. IQI/Jic can be a novel strain to elucidate the relationship between the presence of Thy B and disorders of the immune system.