The subpopulations of lymphocytes (W3/25+ and OX8+) in the blood, spleen and lymph nodes were analyzed comparatively in rats susceptible (Holtzman) and resistant (Buffalo) to the development of adjuvant induced arthritis (AIA). In naive rats, it was found that the ratio (T helper/T suppressor) was significantly higher in Holtzman compared to Buffalo rats suggesting that the susceptible rats possess less suppressor cells than the resistant. Following induction of AIA (7 and 14 days) the ratio T helper/T suppressor was not significantly different in either strain. When the resistant strain was treated with 25 mg/kg of cyclophosphamide, the T suppressor cells were significantly reduced and this was followed by a strong potentiation of the AIA. Our data suggest that susceptibility to AIA is related to alterations in the T suppressor cell subset.