African Green monkeys were injected (2 x daily subcutaneously for six months) with human GRF(1-44)-NH2 (10 micrograms/kg BW) or a more potent analog, [desNH2Tyr1,Ala15]-hGRF(1-29)-NH2 (2 micrograms/kg BW) to determine the potential of each peptide to induce antibody formation. Blood samples were taken every two weeks, diluted 1:100 and tested for ability to bind radioiodinated hGRF. One animal in the hGRF(1-44)-NH2 group [N = 6] produced low-titer GRF antibodies by 6 weeks (19% binding) and continued throughout the 24 weeks of treatment (average = 50-60% binding). Similarly, one animal in the hGRF analog group [N = 6] displayed low-titer GRF antibodies by 18 weeks (14% binding), with the highest binding observed at 24 weeks (51% binding). Subsequent dilutions (1:1,000 and 1:3,000) of these bleedings confirmed that higher GRF antibody titers were not masked by antibody excess. Dialyzed sera from these two animals did not affect the abilities of hGRF(1-44)-NH2 or [desNH2Tyr1,Ala15]-hGRF(1-29)-NH2 to stimulate GH secretion by rat pituitary cells in vitro. After 20 weeks of treatment, significant GH responses (increased mean GH area under the curve 2.3-2.5 fold and GH peak 3.5-3.7 fold, that of control) were observed following hGRF or hGRF analog injection. Therefore, the low titer GRF antibodies detected in monkey sera during six months of treatment with hGRF or a potent analog were biologically non-neutralizing.