Knee synovium, taken from patients with rheumatoid arthritis at the time of arthroplasty, was studied immunohistologically. Focal perivascular lymphoid infiltrates of different sizes were examined in detail to evaluate changes in cell populations as the infiltrate size increased. T cells formed the largest component of mononuclear cells of all aggregates. The large grade 3 aggregates contained substantial numbers of B cells arranged around a central venule and cells bearing the CD45RA+ phenotype. In contrast, the small grade 1 aggregates contained few B cells and the T-cell population contained relatively greater numbers of CD8+ cells. Cells bearing the CD45RO+ phenotype exceeded CD45RA+ cells in grade 1 aggregates. Detailed analysis of mononuclear cell aggregates of different sizes in the rheumatoid synovium suggests that the composition of each aggregate depends on the total number of mononuclear cells it contains.