The development of arthritis induced in mice by intraperitoneal injection of the non-antigenic mineral oil pristane (2,6,10,14-tetramethylpentadecane) was shown to depend on the presence of CD4+ T cells. Initial experiments assessed the influx of lymphoid cells into the peritoneal cavity of CBA/Igb mice after pristane injection. Both CD4+ and CD8+ cell numbers were maximal around 50 days. Other experiments confirmed our original observation that irradiated pristane-treated mice failed to develop arthritis unless they were reconstituted with spleen cells from normal donors. This finding has been extended by showing that the population of transferred splenic lymphoid cells must contain CD4+ T cells, while CD8+ T cells and B cells were not required for reconstitution. Conventionally housed and hsp 65-immunized animals are known to harbour T cells reactive with hsp 65. In addition, hsp 65-immunized mice are resistant to the development of pristane-induced arthritis (PIA). Thus, additional experiments assessed the population of splenic T cells activated and proliferating against mycobacterial 65,000 MW heat shock protein (hsp 65). In cultures of purified splenic T cells derived from both conventional and hsp 65-immunized mice, removal of CD4+ T cells significantly reduced the proliferative response to hsp 65, while removal of CD8+ T cells often enhanced the response. These proliferative responses were also shown to be major histocompatibility complex (MHC) class II restricted. The present findings demonstrate that PIA is CD4+ T-cell mediated, and immunodominant environmental antigens such as hsp 65 activate this population of lymphocytes. The CD4+ hsp 65-reactive population may be pathogenic or protective in PIA, depending upon the route of sensitization.