Abstract
An antiserum to a serum protein from female ACI rats was produced in DONRYU rats by alloimmunization. The serum protein reacting with this antiserum was shown to be a sex-innuenced esterase which is identified by zymogram techniques. Evidence for this is as follows: this protein migrated in the albumin region on agarose gel immunoelectro-phoresis and was not present in sera from mature males; all rats possessing the protein were sex-influenced esterase positive; the activity of the sex-influenced esterase in sera specifically disappeared after treatment with this antiserum. The attempt in ACI rats to produce antiserum to the esterases from DONRYU rats was unsuccessful.