Immunoglobulin G (IgG) antibodies to interleukin 1α (IL-1α) are frequently found in the sera of healthy human individuals. The effects of these autoantibodies on receptor binding and biological activities of human IL-1 were tested. Using the murine T-lymphocyte line NOB-1, human thyrocytes and human foreskin fibroblasts, the antibodies competitively inhibited the biological activity of human recombinant IL-1α (rIL-1α). The degree of inhibition correlated with ¹²⁵I-rIL-1α binding to IgG in different immunoglobulin preparations and in individual sera. These antibodies also neutralized the IL-1 activity of isolated membrane fragments and lysates of human blood monocytes activated by lipopolysaccharide. In contrast, the supernatant IL-1 activity was not affected. Stronger inhibition of biological activity and cell binding of ¹²⁵I-rIL-1α was obtained with NOB-1 cells than with human thyrocytes. The antibodies failed to interfere with the biological activity of rIL-1β. It is concluded that IgG autoantibodies to IL-1α in the sera of healthy humans selectively inhibit the biological activity of the soluble and membrane-associated forms of IL-1α in vitro, and that the degree of biological inhibition afforded by these antibodies depends upon the target cell.