A synthetic decapeptide (SP) was used to produce a murine monoclonal antibody specific for the human IgG3 molecule. Recognition of the IgG3 determinant is heat- and fixation-sensitive in ELISA and immunoenzyme cytology, respectively. The antibody specifically recognizes a sequence from the hinge region of IgG3, but only when subtle alterations in the conformation are induced by mild heating (greater than 40 degrees) and subsequent stabilization by means of electrostatic interactions in solid-phase assays or by fixation with formalin acetic acid mercury chloride. The structure of the human IgG3 molecule is especially sensitive to microenvironmental influences, as can be concluded from its behaviour under various physicochemical conditions. To this, we add that the immunogenic determinants in the structure of relatively flexible parts of this protein can be severely altered by the routine application of fixation methods. It is shown that these changes can also be of importance in the recognition of antigen by monoclonal antibodies.