EDTA-blood samples derived either from healthy staff or septic patients were investigated for in vitro complement activation during the first 48 h after blood drawing at 4° C and 20° C. For this purpose C3a C3a desArg plasma levels were determined by the ABICAP C3a assay. Within the septic group no complement activation was detectable during the whole observation period. However, if blood from healthy persons was stored for longer than 6 h at 20° C complement activation occurred. The most profound activation was found in EDTA-blood stored for 48 h at 20° C. C3a values in this sample increased four-fold from 56 ± 7 ng/ml to 222 ± 38 ng/ml. From these data we conclude that both immediate cooling of EDTA-blood to 4° C, as well as the immediate separation of plasma as proposed by Mollnes et al. (Clin. Exp. Immunol. (1988) 73, 484), is not necessary for determination of anaphylatoxin plasma values. Storage of EDTA-blood samples for up to 6 h without the need to perform centrifugation should allow anaphylatoxin measurement to become a routine parameter for diagnosis of inflammatory diseases.