WE wish to describe new serological techniques based on changes in buoyant density occurring when antibodies, or antigens, or both, are attached to latex beads. For the density change to be observed, the mass of the attached particles must be an appreciable fraction of the total mass. Using very small carrier beads less than one hundred antibody molecules or one virus particle per bead can be detected. The speed and sensitivity of the method make it directly applicable to the clinically important problem of rapid identification of infectious agents.