Materials for soft tissue augmentation in mammals are prepared by cross-linking blood plasma proteins, preferably with a zero-length cross-linking agent. The cross-linked blood plasma proteins can be dialyzed and autoclaved. Such materials are non-antigenic, exhibit decreased allergic response, and have increased longevity with respect to proteolytic attack from natural proteases. The appearance and/or feel of soft tissue defects and/or imperfections in skin can be improved by injecting such materials into an intradermal compartment of a patient's skin.