A significant dependence between different ionic strength and pH value of virus suspension on one hand and the haemagglutinin (HA) content as determined by single-radial-immunodiffusion (SRD) test on the other hand was observed after cleavage of influenza virus recombinant NIB-6 (H1N1) with sodium lauroyl sarcosinate (SLS). In contrast, no such relationship was found when the HA Of NIB-4 (H3N2) recombinant strain was determined.