Abstract: With the method developed by our research, using agar without neutral red, plaques were picked under an inverted microscope, we plaque-purified 3 strains H5N1 subtype AIV chicken embryo isolates and obtained thirteen plaque-purified strains with marked difference in size and shape. Our research indicate addition of suitable amount of trypsin could facilitate plaque formation with marked increase in plaque diameter and plaque-forming units, its optimal concentration being 50 μg/ml, when the concentration of calf serum is 2%. TCID₅₀, EID₅₀ and LD₅₀ testing were done on embryo isolates and the plaque-purified strains, when It was found in the study that for the size and shape of purified strains which came from the same embryo isolate exist notable difference, the larger the plaque the higher the virulence, and that the virulence of the largeplaque strains approached that of the embryo isolates, which indicate that virus particles producing large plaques were dominant in embryo isolates. Analysis of the nucleotide sequences and deduced amino acid sequences of HA genes of the 13 plaque-purified strains revealed that the cleavage site amino acid sequences of the HA protein were all conforming to structures assigned to the molecular characteristics of highly pathogenic notifiable avian influenza virus.

Key words: AIV; H5N1 subtype; plaque clone; biological characteristics