A group of 109 healthy adult blood donors was examined to determine the incidence of anticapsular antibodies capable of eliciting a specific capsular reaction when tested against an encapsulated strain ofStaphylococcus aureus. At the same time the carrier rate for naturally occurring encapsulatedS.aureusvariants was determined. Of the healthy blood donors 80.7% possessed anticapsular antibodies in sufficient titer to elicit a specific capsular reaction. Almost half the carriers of staphylococci carried naturally occurring encapsulated strains ofS.aureus. In a group of subjects below the age of 13 years, a lower incidence of anticapsular antibodies was found.