The gingival crevicular fluid of a patient(s) with marginal periodontal disease contained an activity inhibitory to interleukin-1 (IL-1). The inhibitory activity could be detected after the depletion of IL-1 alpha by the use of a specific antibody (anti-human recombinant IL-1 alpha monoclonal antibody)-conjugated Sepharose column. The inhibitory activity was not due to a toxic effect on the thymocytes since IL-1 alpha-depleted gingival crevicular fluid did not affect the incorporation of [3H]thymidine in either the presence or absence of concanavalin A. The inhibitory activity was exerted against both IL-1 alpha and IL-1 beta. The inhibitory factor did not have any effect on IL-2-induced proliferation of concanavalin A-activated spleen cells. The inhibitor was heat labile. Gel filtration on a Superose 12 column revealed the IL-1 inhibitor to have two major peaks, one in the molecular size range of 12 to 14 kDa and the other below a molecular size of 10 kDa.