Polyarthritis may result from the haematogenous distribution of arthritogenic effector lymphocytes that emerge in the efferent lymph and pass through the thoracic duct (TD) to the circulation. We therefore examined whether TD cells collected from rats in the late prodrome of adjuvant-induced arthritis (AA) could transfer polyarthritis adoptively and whether these cells included a subpopulation of arthritogenic cells that could be identified phenotypically. Unfractionated TD cells collected from donor rats 9 days after adjuvant inoculation were injected intravenously into normal syngeneic recipients in numbers equivalent to the overnight harvest from a single donor. TD cell subpopulations, equivalent in number to proportions in the same inoculum, were prepared by negative selection. Unfractionated TD cells transferred polyarthritis without in vitro stimulation or conditioning of recipient animals. Abrogation of arthritogenicity by depletion of alpha/beta TCR(+) cells showed that the polyarthritis was transferred by T cells. Negatively selected CD4(+) but not CD8(+) TD cells transferred AA. An arthritogenic subpopulation of CD4(+) T cells, enriched by either negative or positive selection, expressed the activation markers CD25 (IL-2 receptor alpha), CD71 (transferrin receptor), CD134 (OX40 antigen) and MHC class II. Cells expressing these markers were more numerous in TD lymph from arthritic rats than in lymph from normal rats and they included the majority of large CD4(+) T cells. Thus, arthritogenic effector T cells bearing activation markers are released into the central efferent lymph in the late prodrome of AA. Recruitment of these arthritogenic cells to synovium probably determines the polyarticular pattern of AA.