We performed a comparative evaluation of the immune status, focused on the T-cell system, of euthymic rat strains in which the nude mutation had been introduced. From 10 institutes, we sampled 12 groups of euthymic rats at ages of 1 1/2-2 months and 1/2 year. We analyzed weight of body, spleen and thymus; antibody response and delayed-type hypersensitivity response to ovalbumin immunization; and (immuno)histopathology of spleen, lymph nodes, and lymphoid tissue along the gastrointestinal tract. In the spleen morphometric analysis was done of the periarteriolar lymphocyte sheath (using the antibody R73 recognizing the alpha beta-T-cell receptor) and of the red pulp (using the antibody ED2 recognizing red pulp macrophages). For almost all parameters tested, statistically significant differences between the groups (origin of the animals) were observed. A cluster analysis on the basis of body weight, spleen weight, and morphometric data of spleen did not yield clusters with a different composition among animals from individual groups. Based on the antibody response to ovalbumin, clustering revealed groups of "fast-and-high", "slow-and-low", and "intermediate" responders. The various groups differed in location within these clusters, i.e. the speed and extent of the immune response depends on the background euthymic strain. Considering the microbiological status assessed by serology, a variation was found both in post-infection state at entrance in the study, and in primo-infection associated with a rise in antibody concentrations during the study. These states showed no negative effect on anti-ovalbumin reactivity. Rather, the response in primo-infection to Rat Corona Virus, Sendai Virus, and Pneumonia Virus of Mice was the highest in animals clustered as "fast-and high" responders to ovalbumin.