The sera of 12 patients with mechanical trauma were studied to determine C3 levels and activation. The injury severity score (ISS) was then related to serum C3 levels and activation. It was found that in the immediate postinjury period, serum C3 activation occurred in cases where ISS was ≥ 12. The mean ISS of patients with complement activation was 25.2. In comparison, in patients with nonactivation of complement, the mean ISS was 9.5 (p<0.05). Serum C3 levels were inversely related to ISS. The mean serum C3 level of patients with ISS ≥12 was 73.3 mg% and mean serum C3 level with ISS ≤12 was 109.4 mg%. This difference was again statistically significant (p<0.05). There is indication that complement depletion occurs in the immediate post injury period in moderate to severe injury (ISS ≥ 12). This finding could explain, in part, the immunosuppressive effect of trauma and can be used as a marker to predict possible septic complications.