Immunocompetent cell functions were evaluated in spontaneously hypertensive rats (SHR). Hematological studies revealed decreased absolute numbers of lymphocytes and increases number of polynucleic cells in the peripheral blood of SHR. The SHR had a reduced number of immature T lymphocytes in their thymuses in comparison with an original strain of Wistar rats, as detected by the rosette formation test with guinea pig erythrocytes. The antibody response to sheep red blood cells (SRBC) of the 3-month-old SHR was profoundly depressed and was about one-teeth that of the Wistar rats. Cell cooperation experiments suggest that the T lymphocytes of the SHR were selectively impaired in antibody responses to SRBC in cooperation with B lymphocytes. B lymphocytes from the bone marrow of the SHR were not affected and produced normal numbers of plaque-forming units. Cyclophosphamide treatment, which selectively depletes suppressor T lymphocytes, did not enhance the delayed-type hypersensitivity response to SRBC in SHR. This may rule out the possibility of the involvement of the suppressor mechanism in the T cell depression of the SHR.