The effects of the following parameters on the immunologic specificity of delayed and immediate hypersensitivity reactions were investigated in the guinea pig using the picryl and p-toluenesulfonyl systems: (a) the contribution of the carrier protein, (b) the effect of the number of hapten groups per molecule of the immunizing and challenging antigens, and (c) the effect of interposing a 6 carbon chain (epsilon-aminocaproic acid) between the hapten and its usual attachment to the lysine epsilon-NH(2) groups of the carrier protein. It was found that induction of delayed hypersensitivity was accomplished equally well with both lightly and heavily coupled conjugates. Sensitized animals which gave strong delayed reactions to the immunizing conjugate cross-reacted poorly or not at all to (a) conjugates of the same hapten with a different carrier protein, or (b) conjugates differing from the immunizing conjugate by having an epsilon-aminocaproyl chain interposed between hapten and its attachment onto the carrier protein. Animals sensitized with either lightly or heavily substituted conjugates exhibited strong delayed reactions to both conjugates, but more intense reactions to the immunizing conjugate were always observed. In contrast to the marker carrier specificity exhibited by the delayed hypersensitivity reactions, immediate hypersensitivity reactions, (specific precipitation, Arthus, and PCA reactions) could be elicited equally well with hapten conjugates of all carrier proteins, as well as with conjugates containing epsilon-aminocaproyl chains interposed between hapten and the carrier protein, provided the number of hapten groups per molecule conjugate was sufficiently high. Both in inducing antibody response and in provoking immediate hypersensitivity reactions, heavily substituted conjugates were considerably more effective than were lightly substituted conjugates. Alternative explanations for these observed differences in specificity between immediate and delayed hypersensitivity reactions are discussed.