Direct cellular interactions, involving adhesion structures like lymphocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1), play a critical role in the initial stages of T-cell-dependent B-cell activation. However, the relevance of cellular contact in later, lymphokine driven stages of B-cell stimulation is less well understood. We have here studied the ability of different lymphokines [interleukin (IL)-1 alpha, IL-1 beta, IL-2, IL-3, IL-4, IL-6 and interferon-gamma (IFN-gamma)] to stimulate adhesion processes as well as proliferation of highly purified tonsillar B lymphocytes. None of the lymphokines were by themselves able to induce aggregation in resting B cells but, when added together with anti-IgM, IL-4 and to a lesser extent IL-2, promoted the formation of large, dense aggregates which were macroscopically visible after 3-4 days in culture. Addition of anti-LFA-1 antibodies (anti-CD11a or CD18) completely inhibited the lymphokine-promoted aggregation, indicating that cluster formation was mediated by LFA-1. Fluorescence-activated cell sorter (FACS) analysis showed that the expression of both LFA-1 and ICAM-1 increased after stimulation with IL-4 as well as with IFN-gamma. However, in contrast to IL-4, IFN-gamma did not enhance cellular aggregation, suggesting that qualitative rather than quantitative changes in LFA-1/ICAM-1 promote aggregation. Although anti-LFA-1 antibodies inhibited aggregation of both IL-2- and IL-4-stimulated cells they did not inhibit proliferation. In contrast, in IL-4-stimulated cultures inhibition of cell contact resulted in a significantly increased proliferation. Furthermore, IFN-gamma-stimulated cells responded with proliferation in the absence of aggregation. Taken together, the findings suggest that LFA-1-dependent cellular contact plays a minor role in lymphokine driven B-cell proliferation. The possible importance of aggregation in B-cell differentiation is discussed.