In the hemagglutination-inhibition test, simple relationships exist between units of hemagglutinating virus and hemagglutination-inhibition antibody titers. Equations have worked out which allow estimating antibody titers for different virus doses, provided all other parameters are kept constant. They are applicable when the isofixation lines in log-log plots as obtained in box titrations either are positioned at angles of 45degree or deviate from this value. Using lower or higher doses of hemagglutinin in the test, the coefficient of variation of the estimated antibody titers was found to be approximately 4 per cent. This procedure is useful in a number of virus-cell-serum systems and allows comparisons of antibody titers obtained in different laboratories.