Lipoxygenase enzymes isolated from soya bean, potato and egg-plant are antigenic, and precipitate with their specific antibodies. All three enzymes have common determinants, since there is cross-reaction among them. Each of the enzymes is strongly inhibited by antibodies of soya bean and potato lipoxy-genase. Some of the determinants are heat stable (to 100 °C) and can compete after heat treatment with the same determinants of the natural enzyme for the same antibody. The peptides which carry the determinant to which the antibody is bound, causing inhibition of enzymatic activity of soya bean and egg-plant lipoxygenase, have been isolated. It is likely that the active site is not identical with any of the antigenic determinants, but it is certainly close to one or several of them. Thus the specific antibody causes steric hindrance of the active site.