Antisera produced in rabbits to 37 rhinovirus (RV) types have been examined for antibody to the 36 corresponding heterologous types. Reciprocal neutralization was noted between RV types 2 and 49 and RV types 13 and 41. Five additional monotypic rhinoviral rabbit antisera neutralized one heterologous rhinovirus. Neutralizing antibody titers against heterologous RV serotypes were similar to those shown in the RV 1A, 1B and RV 9, 32 reciprocal cross-reactions, and would not be expected to result in false identification of serotypes. Comparison of neutralizing antibody titers and neutralization rate constants in serial serum specimens of immunized rabbits showed that antibody response to the immunizing RV type and the cross-reacting type followed a similar time pattern.