A protein that binds spermine specifically was separated from normal rabbit serum by affinity chromatography. Immunoelectrophoresis, the Ouchterlony immunodiffusion test, and gradient gel electrophoresis indicated that this protein has immunoglobulin characteristics and consists of several populations of antibodies to spermine. These were sequentially released from Sepharose-spermine gel by step-wise elution with solutions ranging in pH from 4 to 1. The binding constants varied from 5.0 x 10(8) to 11.1 x 10(8) liters per mole. These globulins did not react with monoacetylputrescine, L-ornithine, L-lysine, and histamine. Negligible cross-reactivity was detected with spermidine, putrescine, N8-monoacetylspermidine, cadaverine, and diaminopropane. Since perturbations in polyamine metabolism have been identified in several diseases, the study of extracellular polyamine homeostasis may reveal an important regulatory function for this protein.