The soluble E-receptor (SER) of lymphocytes that is related to CD2 was detected in human plasma and serum using immunoelectrophoresis with sheep antiserum. All plasma samples (n=18) demonstrated reactivity with antiserum, whereas the reactivity of the corresponding sera remained low or undetectable. The depletion of SER in clotting is associated with fibrinogen, as shown by crossed-affinity immunoelectrophoresis with antisera to plasma proteins. The SER-associated fibrinogen was purified and analysed by the SDS-polyacrylamide gel electrophoresis and immunoblotting. A band close to 66 kDa was detected with monoclonal antibodies to CD2. The association of CD2 and other soluble receptors with fibrinogen via domains is suggested. It is recommended that the fresh plasma, not serum, should be used to study circulating receptors because coagulation may appreciably diminish their physiological level in blood samples.