Opossum pouch young were immunized with bacteriophage (f2, øx-174, and T4), haptens (2,4 dinitrophenyl (DNP), fluorescein conjugates (FTC), and 1-dimethylaminophthalene (DNS)), or proteins (ribonuclease (RNAase), sperm whole myoglobin, and lysozyme). The haptens were placed on three carriers (bovine serum albumin, hemocyanin, or bovine γ-globulin. The sera obtained from animals at sacrifice 2 weeks after immunization were studied by bacteriophage neutralization with either the intact organisms or T4 coupled to one of the haptens or to one of the proteins. When the levels of antibodies were compared with the age of the pouch young, a distinct hierarchy of responsiveness was observed. The order of appearance of responsiveness was: bacteriophage f2, DNP, bacteriophage øx-174, FTC, bacteriophage T4, and RNAase. This hierarchy could not be explained as a difference in sensitivity of the assay, since the assays were of similar sensitivity in each case.