Human plasminogen and plasmin preparations have been analyzed and compared for their antigenic properties. For the evaluation of such preparations, antisera to Kline plasminogen, prepared in rabbits, were used. Kline plasminogen revealed the presence of three distinct components. One of these was identical to gamma globulin of serum by both chemical and immunological criteria while the other two were in the beta-globulin mobility category. One of these latter could be identified with the proenzyme, plasminogen, itself; the other remained an unidentified component which could not be related to the proenzyme. Plasmin showed a similar pattern except for the absence of the gamma-globulin constituent. The two components of plasmin were antigenically identical to the corresponding components of plasminogen.