The antigenic esterase VI A of the human gastric mucosa was compared with extracts of the gastric mucosa of rabbit, guinea pig and rat by means of gel electrophoresis and immunological methods. In all animal extracts we observed several carboxyl esterases with different mobilities. One esterase with cathodical mobility could be compared with the human esterase VI A. Other common antigenic determinants in the human esterase VI A and the esterases found in extracts of guinea pig and rabbit could be detected so far when using heteroimmune sera against (a) the isolated human esterase VI A, (b) extracts of gastric mucosa of guinea pig, and (c) extracts of gastric mucosa of rabbit. Esterases of man, guinea pig and rabbit also have their own antigenic components that are specific to each species. No immunologic relationship could be elaborated with extracts of the gastric mucosa of rat.