Abstract.A monoclonal IgM (IgM Re) was found to react with stored human red blood cells (SHRBC) and human low density lipoprotein (LDL). IgM Re Fab fragment was reactive. SHRBC antigen was present, in a hidden state, in all fresh human ABO erythrocytes studies; it was not present in fresh or stored animal RBC. Cross reactivity of SHRBC and LDL antigens against IgM Re and IgM Re Fab was demonstrated.In contrast with SHRBC antigen, LDL antigen has no species specificity and was also found in rabbit and rat LDL. Furthermore, cross‐reacting soluble inhibitors were also found in human saliva, and in a small molecule fraction issued from human and rabbit serum. The IgM Re also reacted with human and animal glomerular membrane.These findings are suggestive of two closely related but not identical antigens: a human specific SHRBC antigen and a LDL‐associated, SHRBC‐like antigen, which is a secreted, non‐human specific substance. The former is part of the human RBC structure, the latter is a soluble molecule which may be found in saliva and in serum where it may be free or bound to LDL; it also binds to glomerular me