Hydrolysis of Ripley IgG at pH 4.5 and pH 8.0 by leucocytes isolated from human abscesses gave rise to Fab fragments which showed an antigenic specificity resembling that characteristic of Fab fragments produced with the enzymes subtilisin, chymotrypsin and bromelin. At neither pH did the hydrolyses give rise to Fab fragments with an antigenic specificity characteristic of Fab fragments produced by the enzymes pepsin, papain or trypsin.