The aim of the present review has been to describe some immunobiological characteristics of the athymic nude rat and on the basis of these to describe the possibilities of establishing non-antigen-specific T-cell reactivity and inducing antigen-specific immunity. After a brief introduction, some general characteristics of the autosomal recessive rnu/rnu rat are outlined. This athymic mutant normally has a longer lifespan than athymic nude mice and is easier to breed. Inbred nude rats are now available with several rat strain backgrounds. Since the rnu/rnu rat is athymic, the morphology and function of the thymus are briefly described. The thymus has two main functions: production of T lymphocytes and production of thymic hormones. The intrathymic T-cell ontogeny is described along with the two thymocyte selection mechanisms, positive and negative selection. Different thymic hormones are mentioned and their possible function outlined. However, thymic hormones still do not play an important role in immunotherapy. There are some striking differences in the morphology of lymphatic tissues of athymic and euthymic rats. The thymic area in nude rats consists only of fat and clusters of epithelial cells. Little research has been performed on bone marrow of athymic nude rats but morphologically there seems to be no difference from findings in normal animals. The thymus-dependent areas of peripheral lymphoid organs, i.e. the paracortical area of lymph nodes, the periarteriolar sheet of the splenic white pulp, and the interfollicular areas of Peyer's patches, are severely cell-depleted in the athymic nude rat. The lower lymphocyte content of these organs is not reflected in the lymphocyte counts of peripheral blood, but the lymphocyte counts of thoracic duct lymph are much lower than those found in euthymic animals. Few T-lymphocyte marker-positive cells are found in the athymic nude rat, and in the epithelium of the intestine there are also lymphocytes bearing neither T nor B markers. These cells probably represent the tau delta receptor-bearing cells. Immunologically the athymic nude rat reflects the thymic aplasia. Normal B-lymphocyte function has been found in both in vitro and in vivo tests, whereas the T-cell function is virtually absent both in vitro and in vivo. Non-MHC-restricted cells with killer activity like NK cells are present in the nude rat, and these cells function normally both in vitro and in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)