Patients with familial Mediterranean fever (FMF) who were part of a double-blind trial of daily colchicine as prophylaxis for their disease had leukocyte functions studied while receiving colchicine or placebo. Leukocytes taken from these patients while on prophylactic doses of colchicine produced normal quantities of leukocytic pyrogen, ingested bacteria normally, and migrated normally in chemotatic chambers. In addition these patients had normal numbers of circulating T and B lymphocytes as well as normal blastogenic reponses of their peripheral lymphocytes to mitogenic stimuli. The patients on colchicine, however, had significantly fewer neutrophils and monocytes accumulating at skin-window sites 24 hours after the initial abrasion. Because the early phase of the skin-window response was normal in these patients, the decreased late response may be related to a failure to amplify the initial inflammatory reaction. The reduced capacity to generate a normal inflammatory response may account for the failure of these patients to develop full attacks while taking colchicine.