1. The binding rate of PAS to human serum albumin measured by the method of equilibrium dialysis correlate the albumin concentration and inversely correlate the PAS concentration in the range of 50 to 400 mcg/mI.2. The binding rate of PAS to human albumin is the lowest at pH 6.5 and increases according to the increasing alkalinity.3. Almost the same binding rates of PAS to serum protein are obtained either in vivo and in vitro by the method of equilibrium dialysis.4. There is an individual difference in the binding rates of PAS to serum protein even when corrected by the albumin concentration.5. The inverse correlation is proved between the blood level and the acety-lation rate of PAS.6. The inverse correlation is also proved between the binding rate and acetylation rate of PAS.7. There is a significant correlation between the unbound blood level of PAS and'the microbiologically active blood level of INH, and this result may suggest that the same grade of acetylation takes place in the same individual even when the different drugs are given.8. The serum protein binding is one of the most important factors of inactivation of the drug in vivo as well as the acetylation, and it may influence the clinical therapeutic effect of the drugs.