ABSTRACT
An effort was made to determine the optimum concentration of diphtheria toxoid in combination with aluminum phosphate gel in DPT vaccines which may give a safe, potent and economical preparation. The effect of four different concentrations of aluminum phosphate and three different antigenic concentrations of diphtheria toxoid on potency of diphtheria component in DPT vaccine was assessed. A gradual increase in potency was seen with increase in toxoid concentration and a gradual decrease in potency with the increase in aluminum phosphate content. Vaccines made with minimum quantities of toxoid (30 Lf/ml) and aluminum phosphate (3 mg/ml) were found to be highly satisfactory. Vaccines prepared with high antigenic purity toxoid have better potency, as compared to those prepared with a relatively low antigenic purity toxoid.