In this paper the contribution of different accessory molecules to the adhesion of resting, naive and memory CD4+ T cells was examined utilizing a panel of CHO cell transfectants as model antigen-presenting cells (APCs). CD4+ T lymphocytes demonstrated strong adhesion to HLA-DR4 transfected CHO cells co-expressing B7, ICAM-1 or LFA-3 molecules, suggesting that all three adhesion pathways is utilized by resting CD4+ cells. Monoclonal antibodies (MoAbs) against the corresponding receptors on T cells, e.g. anti-CD28, anti-LFA-1 beta and anti-CD2, inhibited completely T-cell adhesion to natural ligands expressed on transfected CHO cells. Pretreatment of CD4+ T cells with NKI-L16 MoAb, which interact with an activation epitope on LFA-1 alpha chain, enhanced adhesion to ICAM-1 but not B7 or LFA-3-expressing CHO cells. Analysis of T helper-cell subsets revealed that memory T cells bound several fold stronger to ICAM-1 expressing transfectants compared to the CD4+ 45RA+ naive T cells, whereas adhesion to B7, LFA-3- and B7/LFA-3-expressing CHO cells was similar in both T-cell subsets. The kinetics of adhesion of naive and memory CD4+ T cells to ICAM-1 was rapid and similar in both subsets. The NKI-L16 MoAb multiplied several times ICAM-1-dependent adhesion in naive compared to memory cells, which enabled the naive cells to reach a similar adhesion level as memory cells. The results suggest that resting naive CD4+ T cells utilize preferentially the CD2/LFA-3 or CD28/B7 adhesion pathways upon adhesion to APCs, while memory CD4+ T cells utilize the CD2/LFA-3, CD28/B7 and LFA-1/ICAM-1 adhesion pathways. The NKI-L16 MoAb-induced upregulation of adhesion involves an increased affinity of LFA-1 for its ligand and not a change in the number of LFA-1 molecules. This is compatible with a view that naive cells express a large number of inactive LFA-1 molecules, whereas memory cells express preferentially activated LFA-1 molecules. The inherent low number of active LFA-1 molecules on naive CD4+ T cells may be important in keeping these cells in a resting state.