The presence of soluble fibrin complexes (SFC) measured by gel filtration of plasma on 4% agarose columns, fibrinogen heterogeneity on 3.5% SDS-polyacrylamide gels and the concentrations of several plasma proteins were evaluated in 39 patients with diabetes mellitus (DM) and 19 matched control subjects. A small but significant increase of SFC was found in DM (p less than 0.01). On individual basis 51.2% of the patients had increased SFC (greater than M + 2 SD of the controls). Polyacrylamide gel electrophoresis of the SFC showed no evidence of cross-linking or proteolysis. Plasma clots formed in the presence of EDTA and trasylol were analysed in SDS-polyacrylamide gels in a normal and two lower molecular weight fibrin bands (band I, II, III). The percentage of band I fibrinogen was in diabetics (65.3 +/- 4.7%) lower than that of the controls (71.8 +/- 4.5%) (p less than 0.01). Fibrinogen levels, antithrombin III, alpha 1-antitrypsin, alpha 2-macroglobulin and plasminogen were significantly increased in DM. We suggest that in DM there is an enhancement of intravascular fibrin formation and accelerated fibrinogen degradation to lower molecular weight forms.