The two-dimensional double immunodiffusion, the so-called 'two-cross' method, is introduced for determination of the size of antigenic subunits released after solubilization of enveloped viruses with non-ionic detergents. The method enables the determination of diffusion coefficients and calculation of relative molecular masses of immunoreacting components without using any standard and without prior isolation, purification or labelling of material to be analysed. The molecular masses of surface antigenic fragments of the Newcastle disease virus (NDV) disrupted by non-ionic detergent Triton X-100 were determined. Two antigenic fragments, the larger having a molecular mass between 350 and 187 kDa and the smaller from 140 to 86 kDa, were released by the action of 0.1-1.0% detergent at 20 degrees C. One fragment, whose molecular mass varied from 210 to 187 kDa, was obtained at 37 degrees C after treatment of NDV with 0.2-1.0% detergent, respectively The detergent disruption of purified NDV is studied for comparison. No difference was found whether purified or NDV in allantoic fluid was subjected to the same detergent extraction.