Two peptides of potential utility for targeting melanoma cells, alpha-melanocyte-stimulating hormone (alpha-MSH) and its more potent analogue [Nle4,D-Phe7]-alpha-MSH, were radioiodinated in 45-65% yield using N-succinimidyl 3-[125I]iodobenzoate (SIB). To determine whether this labeling method resulted in improved in vitro and in vivo characteristics, these peptides also were labeled with 131I by direct iodination with the iodogen method. For alpha-MSH, the rapid tissue clearance of both radionuclides in mice was consistent with rapid degradation of the peptide; however, significantly lower levels of 125I were observed in thyroid and stomach, reflecting a greater inertness to deiodination. More extensive comparisons were performed with [Nle4,D-Phe7]-alpha-MSH. The in vitro binding of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH (prepared using SIB) to the murine B-16 melanoma cell line, 34.1 +/- 4.7%, was more than twice as high as that for [Tyr2(131I),Nle4,D-Phe7]-alpha-MSH (15.0 +/- 0.1%), and its KD was more than 10-fold lower than that for conventionally labeled peptide (10 +/- 5 versus 140 +/- 14 pM). The normal tissue clearance of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH in mice was faster than that of [Tyr2(131I),-Nle4,D-Phe7]-alpha-MSH. The 19-40-fold lower activity concentrations of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH in tissues accumulating free iodide (thyroid and stomach) suggest a greater inertness of this peptide to deiodination. The primary urinary catabolite of [Nle4,D-Phe7, Lys11-(125I)IBA]-alpha-MSH was the lysine conjugate of iodobenzoic acid, whereas radioiodide was the chief catabolite generated from [Tyr2(131I),Nle4,D-Phe7]-alpha-MSH. We conclude that further evaluation of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH for targeting alpha-MSH receptors is warranted and that SIB may be a useful method for the radioiodination of peptides.