Leumedins are small organic molecules with anti-inflammatory properties in vivo. We report here that leumedins inhibit the CD11b/CD18 alpha M beta 2 (Mac-1)-dependent adherence of neutrophils to serum proteins. The activation of neutrophils leading to adherence via Mac-1 is associated with an increase in cell surface Mac-1 level, and with an increased affinity of Mac-1 for adhesion partners. Inhibition of neutrophil adherence by leumedins does not require blocking the recruitment of Mac-1 from intracellular granules to the cell surface. Furthermore, leumedins do not block the expression on Mac-1 of the epitope for an "activation-specific" antibody (CBRM1/5). Time course studies show that leumedins inhibit adherence by targeting an event which occurs concurrently with changes in Mac-1 level and induction of the CBRM1/5 epitope. Therefore, leumedins block an unknown process which is permissive for Mac-1-dependent adherence.