IT IS RECORDED that William J. Mayo, in recalling the early days at Rochester, said of himself and his equally illustrious brother: "We were very green and we knew it." The latter half of this sentence furnishes one key to their accomplishments. When the invitation was received to give this lecture, the thought occurred to me that perhaps I might best fulfil the purpose of its distinguished founder were I to present a subject the fundamental knowledge of which is still in bud. Surgeons are not unaware of the extent of ignorance on this subject; therein lies the hope of the future, just as their awareness of ignorance determined in part the growth in skill and knowledge of the Mayo brothers. The subject which I have chosen is the reaction of tissues to foreign bodies. My hope is to define within the limits of my capacity the boundaries of our