SS blood group tattoo SS blood group tattoos ( ) were worn by members of the Waffen - SS in Nazi Germany during World War II to identify the individual ' s blood type . After the war , the tattoo was taken to be " prima facie " , if not perfect , evidence of being part of the Waffen - SS , leading to potential arrest and prosecution . Description and purpose . The SS blood group tattoo was applied , in theory , to all Waffen - SS members , except members of the British Free Corps . It was a small black ink tattoo located on the underside of the left arm , usually near the armpit . It generally measured around 7mm ( 0 . 28 inches ) long , and was placed roughly 20 cm ( 8 inches ) above the elbow . The tattoo consisted of the soldier ’ s blood type letter , either A , B , AB or O . The discovery of the rhesus factor had been made in 1937 , but was not fully understood during World War II , so was not implemented . In the early part of the war tattoos were printed in Gothic - style lettering , while later on they were printed in Latin - style . The purpose of the tattoo was to identify a soldier ' s blood type in case a blood transfusion was needed while unconscious , or his " Erkennungsmarke " ( dog tag ) or " Soldbuch " ( pay book ) were missing . The tattoo was generally applied by the unit ' s " Sanitäter " ( medic ) in basic training , but could have been applied by anyone assigned to do it at any time during his term of service . Usage . Not all Waffen - SS men had the tattoo , particularly those who had transferred from other branches of the military to the Waffen - SS , or those who transferred from the " Allgemeine SS " , the " General " or non - military SS . Some non - SS men also had the tattoo : if a member of a branch of the " Wehrmacht " was treated in an SS hospital , they would often have the tattoo applied . Although the tattoo was widely used in the early war years , over the course of the war it was gradually applied to fewer and fewer soldiers , and towards the end of the war having the tattoo was more the exception rather than the rule . The application of the tattoo to foreign volunteers was apparently an issue of contention , with some , such as the " British Free Corps " not required to have it , while other foreign units did not object . Very little specific information exists regarding the tattoo and foreign units , but it is claimed by some that the men of the 33 . Waffen - Grenadier - Division der SS " Charlemagne " had the tattoo applied . Johann Voss of the 6 . SS - Gebirgs - Division " Nord " and author of " Black Edelweiss " , did not have the tattoo applied although the rest of his training company did , as he was visiting his father on that particular day . Postwar . When the war ended , the Allies were keen to catch all Waffen - SS members on account of the high volume of war crimes committed by some units . The blood group tattoo helped greatly in identifying former members , leading to their prosecution and in some cases their execution . Because of the lack of perfect consistency between having the tattoo and having served in the Waffen - SS some SS veterans were able to escape detection , while others who had not served in that branch of the German military were falsely categorized as having done so . Some members of the SS who evaded capture in part because they did not have the blood group tattoo included Josef Mengele and Alois Brunner .