Pristane injection caused mediastinal lymphadenopathy in arthritis-susceptible DBA/1 mice, but not in arthritis-resistant DBA/2 mice. Early DBA/1 mediastinal lymph node changes were characterized by the accumulation of surface Ig+ cells and a CD8+ (Lyt 2) lymphocyte population, causing an inversion of the CD4/CD8 ratio. Depressed mitogen responses and the appearance of a nonspecific suppressor cell population were coincidental with the CD8+ cell accumulation. Prior to the development of overt clinical arthritis, an expansion of CD4+ (L3T4) lymphocytes displaced CD8+ as the predominant phenotype in mediastinal node. Mitogen responses were restored and suppressor cell activity was abrogated concomitant with the expansion of the CD4+ cell population.