We investigated to what extent the adherence molecule ICAM-1 is important for the accessory cell function of cells of the monocyte lineage. A monoclonal antibody to ICAM-1, 7F7, inhibited the accessory cell function of human peripheral blood monocytes cultured in the presence of serum for 1-3 days. Monocyte, cultured for 4-7 days still expressed ICAM-1 but their accessory cell function was no longer inhibited by the antibody 7F7. Dendritiform accessory cells, obtained by culture of peripheral blood monocytes in serum free medium for 1-2 days also expressed ICAM-1 and their capacity to stimulate T-cells was also abrogated by antibody 7F7. These findings suggest that ICAM-1 on the surface of accessory cells of a certain differentiation stage seems to contribute to the capacity of these cells to stimulate T-cells but, that in some situations accessory cell function can occur normally without requiring ICAM-1 dependent cell to cell interactions.