AbstractImmunization by multiple intradermal injections of rabbits with pepsin and pepsinogen elicited a large amount of antibodies against these proteins. The antibodies were highly cross‐reactive even at pH 7.4 as measured by precipitin and direct binding techniques. The degree of cross‐reactivity was reduced after passage of the antisera through Sepharose columns containing covalently bound heterologous antigen.The cross‐reactivity between pepsin and pepsinogen was quantitated by several methods: (a) by precipitin reaction, (b) by measuring the amount of antibodies obtained after passing sera on Sepharose columns to which the heterologous antigen was covalently bound, (c) by measuring the inhibition of the binding of125I‐labeled antigen to antisera by unlabeled antigens and (d) by measuring the amount of heterologous125I‐labeled antigen which could be added to an antiserum already saturated with the homologous125I‐labeled antigen.The results indicated that anti‐pepsinogen antisera contained a class of antibodies which could only bind pepsin and not the homologous antigen, pepsinogen. These results suggest that in rabbits immunized with pepsinogen, antibodies were produced against determinants which are not present in nati
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