Neutralization kinetic analysis for appropriate seven strains of herpes simplex virus (HSV) was performed using guinea pig antisera. Six strains among those were different in the diagnosis or plaque sizes in African grivet monkey kidney (GMK) cells. In the first place, the reliability of neutralization rate constant (K values) was examined. K values of each antiserum against every virus strain were determined under the suitable condition and the normalized neutralization constants (NK values) were calculated for antigenic comparison. The average NK value against the standard HF strain of six heterologous antisera was 41, which was the lowest of those against the virus strains tested. On the other hand, Z strain was neutralized most rapidly by heterologous antisera. The average NK value was 94 which was the highest value. For the serological differentiation by NK value, anti-HF serum was unsuitable. On the contrary, NK values of anti-Z serum were the lowest and the most separate values against the heterologous strains, so that it will be the most excellent for serological differentiation. The results were not so successively feasible as might have been expected. The some reasons were discussed.