Protein preparations from commercial suppliers are contaminated with bacterial endotoxins. The continued use of these preparations indicates that many researchers are unaware of this, and they may attribute all observed effects to the proteins themselves. Intravitreous injection of bovine serum albumin has an initial inflammatory effect on the rabbit eye which occurs before an immune reaction to the antigen itself can develop. This direct inflammatory effect can be fully accounted for by endotoxin-like contaminants which are present in protein preparations obtained from commercial suppliers. A pharmaceutical (U.S. Pharmacopeia) serum albumin preparation contains no detectable endotoxin, and has no initial inflammatory effect on the eye. Since endotoxins, even in minute amounts, have a variety of effects, the use of such contaminated protein preparations in biological research can lead to erroneous conclusions and should, therefore, be avoided.