Synopsis The substantivity of p-amino benzoic acid (PABA) and two of its esters (Escalol 507 © and Amerscreen "P"©), in an alcoholic vehicle, has been studied both in vitro and in vivo in order to establish the possible correlation between the two methods. Eight healthy volunteers participated in the in vivo method. Solutions of the sunscreen substances were prepared in isopropanol (IPA): PABA at 5%, and Escalol 507 © and Amerscreen "P"©, both at 2%. In order to study the substantivity of each of the sunscreen substances to the skin, the subjects placed their hands in the solutions. The hands were then rinsed with water and extracted with IPA at 50øC to quantitate the amount of substance retained by the skin. A crossover design was used; all the subjects participated in experiments with each of the substances at one week intervals. In the in vitro experiment, keratin powder obtained from hard, insensible tissues (human calluses of the feet) was used. The keratin was kept in an ethanol solution of the sunscreen for 72 hr, then treated with water for 48 hr, and finally extracted with ethanol at 50øC for 24 hr in order to determine the amount of sunscreen retained by the keratin. The sunscreen concentrations in the different solutions in both in vitro and in vivo methods were determined by spectrophotometry. The in vivo percent substantivities (i.e. percentage of sunscreen deposited which was not rinsed off ,by water) obtained in the study were: PABA = 0.30 _+ 0.05; Escalol 507 © = 57.96 --- 0.63; Amerscreen "P"© = 0.46 + 0.08. In the in vitro method the following values were obtained: PABA = 0.30 +-- 0.02; Escalol 507 © = 50.84 + 0.66; Amerscreen "P"© = 0.47 + 0.03. The differences are statistically significant (p < 0.01) and Escalol 507 © is the substance that shows this property in the highest degree.