BLOOD WAS DEPOSITED ON SIX SUBSTRATES: GLASS, WOOL, NYLON, AND THREE TYPES OF PURE COTTON (PLAIN COTTON, PERMA PRESS, AND DENIM), AND THE RESULTING DRIED BLOOD SPECIMENS WERE AGED AT AMBIENT TEMPERATURE UNDER 20 AND 66 PERCENT RELATIVE HUMIDITY CONDITIONS. ANALYSES WERE PERFORMED AT 1, 2, 4, 13, AND 26 WEEK AGING PERIODS. RED CELL ANTIGEN SYSTEMS EVALUATED IN THIS STUDY INCLUDED ABO, MN, RH, KIDD, DUFFY, AND KELL. THE MOST STABLE ANTIGENS AMONG THOSE INVESTIGATED WERE A,B,AND O OF ABO; M, N, AND S OF MN; AND D OF THE RH SYSTEM. THESE VARIANTS WERE IDENTIFIED IN SPECIMENS THAT WERE AGED FOR 26 WEEKS AT THE LOW AND HIGH RELATIVE HUMIDITY CONDITIONS. THE LEAST STABLE VARIANTS ARE ALSO IDENTIFIED. FOUR ENZYME SYSTEMS ALSO INCLUDED IN THIS STUDY WERE AK, ADA, PGM, AND EAP. THE PROBABILITY FOR TWO RANDOMLY SELECTED INDIVIDUALS TO HAVE THE SAME COMBINATION OF VARIANTS WAS CALCULATED FOR EACH TEST PERIOD, BASED ONLY ON THE PERSISTING GENETIC MARKERS INCLUDED IN THE STUDY, USING BLOOD FREQUENCY OF OCCURRENCE DATA IN THE LITERATURE, AND ASSUMING NO ASSAY ERRORS. DISCRIMINATION OF ONE IN 2500 IS POSSIBLE AT ONE WEEK FOR EITHER HUMIDITY STORAGE CONDITION. AT 26 WEEKS, HOWEVER, THE VALUES ARE ONE IN 65 AND ONE IN 28 FOR AGING AT AMBIENT TEMPERATURE UNDER 20 AND 66 PERCENT RELATIVE HUMIDITY LEVELS, RESPECTIVELY. ADDITIONAL FACTORS AFFECTING THE DISCRIMINATION PROBABILITY ARE INCLUDED. (AUTHOR ABSTRACT)