The viral surface proteins of influenza type A viruses, hemagglutinin (HA) and neuraminidase, are subject to antigenic evolution, which leads to the antigenic shift and the antigenic drift". Antigenic drift is a variation within a subtype of the virus, and thought to result from spontaneous point mutations during virus replication followed by the selection of mutants in a partially immune host population. Antigenic drift in the HA molecule is epidemiologically of a great importance, probably because immune mechanisms neutralise virus through interaction with the HA molecule. This paper reports a study which was aimed to investigate the antigenic drift of H3N2 subtype of influenza type A virus from 1968 to 1981. For this purpose, 4 HI monoclonal antibodies against HA of A/Bangkok/1/79 were generated and the HI titers against 36 strains of the H3N2 subtype were studied. Antigenic analysis of naturally occurring variant of H3N2 strains before 1977 was studied by Webster et al." using monoclonal antibodies. We found significant differences between strains isolated before 1976 and after 1977, and some variaties among strains isolated 1977 through 1981. The viral antigen for producing monoclonal antibody-secreting hybridomas and for screening the hybridomas with the enzyme-linked immuno-sorbent assay (ELISA) was a concentrated split (SP) vaccine"' prepared and kindly supplied by Dr. Y. Igarashi, Kitasato Institute, Tokyo. Briefly, the strain A/Bangkok/1/79 RX 73 of influenza virus, a recombinant of A/Bangkok/1/79 and A/PR/8/34, (obtained from Dr. A.P. Kendal, C.D.C., Atlanta. Ge.), was grown in chick embryos and was purified by a Sharpless ultracentrifugation and adsorption to and elution from BaSO, followed by sedimentation through a linear sucrose gradient (24 to 30%). The virus particles were split with Tween 80 and ether, and the water