Thin layer immunoassay (TIA) is a method for analysis and quantification of antigen-antibody reactions on solid surfaces [ 1,2]. Elaboration of TIA was based on the observation that a polystyrene surface coated with a double layer of antigen and antibody is more wettable than a surface coated with antigen only. The increased wettability is visualized as a distinct hydrophilic condensation pattern when the surface is exposed to water vapour [vapour condensation on surface (VCS) technique]. The immunobiological relevance of the large difference in wettability on antigen and antigen-antibody surfaces is largely inknown although the wettability parameter is a fundamental characteristic of a solid surface and its adsorbed molecules. Several of the antibody-mediated immunological effector mechanisms, e.g., opsonization or complement fixation, occur on the surface of bacteria or other micro-organisms. This investigation used the contact angle measurement technique in [3] to characterize and quantitate wettability of antigen and antigen-antibody layers adsorbed on polystyrene surfaces in greater detail. The contact angle measurement technique is most appropriate since, in contrast to the VCS technique used in TIA, it registers objectively and quantitatively the wettability of a solid surface.