Abstract The paper gives a review of the immunogens most frequently used for preparing antibodies against steroid haptens. The structural concepts of immunological specificity and complementarity are applied to a description of the formation of antibodies in immunized animals and the binding mechanisms of steroid-antibody interactions in systems in vitro. Recent experimental finclings show that Ehrlich's lock and key principle is too static for many steroid-antibody complexes and that a more important factor in the binding events is the overall flexibility of these complexes. In this connection a discussion is undertaken of the properties of unsaturated steroid skeletons, the multispecificity of binding sites and further critical factors involved in the formation of the final specificity. Special attention is paid to considerations of the size of steroid determinants in connection with the question of the so-called bridge effect. The relevant expositions of this phenomenon are based in this article more on conformation changes of the steroid ligands than on direct binding interaction of the bridge substituent with the antibody. This interpretation makes it possible to interpret more broadly the experimental facts and gives new stimuli for an improvement of the present strategies of steroid immunoanalysis.