Treatment with peptide hormones leads to the development of specific antibodies (Abs). Structural homology does not prevent induction of high affinity Abs. GRF 1-44 NH2 and GRF 1–29 NH2 are linear peptides. Formation of amphilic secondary structures on cell surfaces can be important for biological action and immune response. Guinea pigs were immunized against 1-44 NH2 and 1-29 NH2. The concentrations, the affinities and the sizes of IgG-Abs-GRF complexes were determined by ultracentrifugation. The native hormone is a fair immunogen which induced specific Ab concentrations of 593 ± 41mg/l after 4 weeks, comparable to insulin and hGH in the same system. 85% of these Abs bind to epitopes of the amino-acid residues 30-44 with high affinity (3.8±0.9 109l/M), only 15% bind to epitopes of sequence 1-29 with lower affinity (6±0.3 108 1/M) . Abs to 1-29 NH2 had lower concentrations (36.4 ± 8mg/l) and lower affinities towards the native hormone (4 ± 0.2 108l/M) and the 1-29 NH2-fragment (3±0.2 108l/M). These data support the concept of amphilic secondary structures. 1-44 NH2 is a divalent antigen which forms 7S and 10S antigen-Ab complexes, 1-29 NH2 forms largely monomeric complexes. One out of 9 children treated with 1-29 NH2 s. c. for 3 ms developed Abs with low titer and low affinity, 2 after 6 ms with low titer and affinity. Samples drawn 6 ms after discontinuation were negativi.