Protein adsorption and complement activation were studied on thin evaporated films of titanium (Ti). The surfaces were cleaned in either a radio frequency (RF) plasma unit, or washed sequentially in trichloroethylene, acetone, ethanol, and water. Both methods resulted in hydrophilic surfaces with low carbon contamination on the outermost oxide ( ~ 11-13 at%). In situ ellipsometry suggested that Ti is an intrinsic coagulation activator in vitro, since significant amounts of factor XII (F XII) and high molecular weight kininogen (HMWK) were found on the surfaces after 1 min incubation in heparin plasma. Ellipsometry, performed after serum incubations ranging from 15 s to 30 min showed that the total amount of serum proteins and the deposition of antibodies to complement factor 3c (C3c) increased with serum incubation time. ELISA methods showed increased levels of free iC3b in serum after 10 min incubation of the surfaces, but no detectable amounts of C3 convertase fractions C4d or Bb. Ellipsometric results indicated, however, an increased deposition of antibodies to C I q and IgG on Ti after short serum incubation times. The combined results indicate that Ti-surfaces initially activate complement through the classical pathway. The activation then continues via a positive amplification loop where increased amounts of C3 are deposited on the surfaces via the alternative pathway.