An improved assay of target components in a sample utilizes specific gravity-altering particles which are attached to the target components by specific antibodies. The attached specific gravity-altering particles are preferably liposomes which will buoy or sink the targets to a common level in the specimen sample when the latter has been centrifuged in a transparent tube. The liposomes can provide an accentuated and more pronounced indication of the presence of the targets in the sample due to their ability to contain many multiples of fluorescent or non-fluorescent dye molecules with minimal steric interference with the attached antibodies' binding ability.