A comparison has been made of PCA induced in mice with rabbit and with mouse antibody. Approximately 50 times more rabbit anti-Ea Ab N than mouse anti-Ea Ab N is required to produce the minimal response. PCA can be induced in mice with homologous antibody at least as early as 15 min and at least as late as 6 hr after the injection of antibody. The duration of sensitivity to PCA with mouse antibody, therefore, is very similar to that reported by Ovary with rabbit antibody. Despite the close phylogenetic relationship of mice and rats, approximately as much mouse antibody as rabbit antibody is required to sensitize the rat to PCA. Cells of Bordetella pertussis do not enhance PCA in the mouse sensitized with either mouse or rabbit antibody.