TO HELP PRACTICING CRIMINALISTS SELECT THE BLOOD GENETIC MARKERS MOST LIKELY TO YIELD SIGNIFICANT RESULTS UNDER VARIOUS AGING CONDITIONS USING PRESENT ANALYSIS TECHNIQUES, THE DETECTABILITY OF THE ABO, MN, RH, KIDD, DUFFY, AND KELL RED CELL ANTIGEN SYSTEMS AND OF THE ADENYLATE KINASE (AK), ADENOSINE DEAMINASE (ADA), PHOSPHOGLUCOMUTASE (PGM), AND ERYTHROCYTE ACID PHOSPHATASE (EAP) ENZYME SYSTEMS WAS STUDIED. BLOOD FROM 12 DONORS WAS DEPOSITED ON GLASS, WOOL, NYLON, AND THREE TYPES OF COTTON (PLAIN, PERMA-PRESS, AND DENIM). EMPHASIS WAS PLACED ON FABRICS, SINCE MORE THAN 90 PERCENT OF BLOODSTAINS USED AS EVIDENCE ARE FOUND ON TEXTILES. THE SAMPLES WERE THEN EXPOSED TO HIGH AND LOW HUMIDITY CONDITIONS AT 20 DEGREES C.; TWO SAMPLES WERE FROZEN AT -20 DEGREES C. UNDER LOW HUMIDITY. TECHNIQUES USED TO AGE THE SAMPLES AND RUN THE ANALYSES ARE DETAILED. THE MOST STABLE ANTIGENS WERE A, B, AND O OF THE ABO SYSTEM, N, N, AND S OF THE MN SYSTEM, AND D OF THE RH SYSTEM. ALL WERE IDENTIFIED IN BOTH HIGH AND LOW HUMIDITY SPECIMENS AFTER 26 WEEKS. SUBSTRATE MADE LITTLE DIFFERENCE. THE MOST UNSTABLE ANTIGENS WERE JK(A) OF THE KIDD, FY(A) OF THE DUFFY, AND K OF THE KELL SYSTEMS, ALL DETECTABLE FOR ONLY ONE WEEK AT EITHER HUMIDITY LEVEL. AK AND PGM ISOENZYMES WERE IDENTIFIABLE AT 26 WEEKS AND ADA AND EAP AT 13 WEEKS FOR HIGH AND LOW HUMIDITY STORAGE. PGM ISOENZYMES MORE STABLE AT LOW HUMIDITY, ADA AND EAP AT HIGH HUMIDITY. FREEZING PRESERVED ALL SPECIMENS BETTER THAN ROOM TEMPERATURE. CHARTS AND TABLES GIVE DATA FOR EACH SYSTEM ANALYZED. A SUMMARY AND GLOSSARY ARE ALSO INCLUDED.