It is imperative that operating personnel and regulatory agencies have secure, advance, and accurate knowledge of the true surface properties of materials to be contacted with flowing blood. The actual devices must be characterized in their sterile, immediate pre-exposure states. Although very sensitive electronic instrumental analysis techniques are available, the requirements for small specimens exposed to high vacuums and energetic beams hinder their use with practical prosthetic implants. Thus, it remains important that classical estimation of the "wettability" of nonphysiologic materials by simple contact angle measurements can document their real surface qualities in the condition and environment of their intended application. These data, acquired quickly and nondestructively, together with confirming laboratory results from internal reflection infrared spectroscopy, scanning microscopy, and X-ray analysis, are the most useful indicators of material safety and effectiveness for blood contact uses. Experience has shown that the critical surface tensions of materials, determined from contact angle measurements in air rather than underwater, are generally predictive of the tenacity of bioadhesion at their boundaries with blood and other living tissues. Overall, it is now recommended that the surface chemistry of all implant materials be characterized before any exposure to flowing blood, as a minimum requirement of good practice.