H L A antigens and membranebound immunoglobulins 297 ral antibodies are elicited is not unique. Benjamini and his collaborators (personal communication) have observed a similar phenomenon when testing the cross-reaction between native lysozyme and its reduced carboxymethylated derivative; similarly, Michaeli* and his colleagues have encountered cellmediated cross-reaction between Ascaris and human collagens, whereas n o humoral crossreactivity between them could be detected. It seems, therefore, that the use of cell-mediated immune response may be o f help in following phylogenetic relationships between proteins which might have diversified during evolution t o such an extent as t o prevent humoral cross-reac tion.The financial support of the Ford Foundation is gratefully acknowledged.