We have studied the effect of the two antibodies, MF 25.1 and GF 26.7.3, on leucocyte function: these antibodies did not mimic the effect of cellular stimuli, but the pre-treatment of neutrophils with MF 25.1 and GF 26.7.3 modulated several cell activities. Both antibodies significantly reduced superoxide anion production in response to various stimuli. Despite similar cellular response MF 25.1 and GF 26.7.3 bound to different antigenic determinants, as shown both by immunoblotting studies and by treating neutrophils first with GF 26.7.3 and subsequently with MF 25.1, and vice-versa; the inhibition of superoxide production corresponded, in fact, to the total of that seen for each antibody. Moreover, in the absence of attractant, measures of locomotion of neutrophils treated with monoclonal antibody showed that 43% of the subjects responded to antibody binding by twofold activation of spontaneous movement. Chemotaxis and lysosomal enzyme release were not affected by antibody treatment.