The effect of polyclonal (anti α and β chain) and monoclonal (anti α chain) antibodies lymphocyte Function‐associated antigen 1 (LFA‐1) on I cell activation was studied. When added at the beginning of activation but not after 24 h or later the antibodies as well as the F(ab')2or Fab fragments of polyclonal antibodies inhibited concanavalin A (Con A)‐induced proliferation, interleukin 2 (IL‐2) production, and the expression of receptors for IL‐2 and transferrin. The inhibitory effect reached a maximum at the same time as optimal proliferation (72 h) Inhibition of proliferation lasted for 5 days or longer, although IL‐2 production was only inhibited during the first 48 h of culture. Receptors for IL‐2 and transferrin were re‐expressed to the original level after 3 days of activation. Addition of external IL‐2 at the beginning of the anti‐LFA‐1 containing culture prevented the inhibition of IL‐2 receptor expression, while inhibition of transferrin receptor expression was unaffected, supporting the conclusion that the‐expression of these two receptors is regulated by partially independent signals. The polyclonal and monclonal anti‐LFA‐1 antibodies also inhibited phorbol ester (PMA)‐dependent OKT3 activation of highly purified T cells. The results suggest that the LFA‐1 antibodies block an early step in the reactions necessary for IL‐2 production, and that the LFA‐1 molecule participates not only in T cell‐accessory cell interaction but also in T‐T interaction durin