Two novel N-terminal fragment pairs of porcine pro-gamma-melanocyte-stimulating hormone (MSH)-(1-103), viz. pro-gamma-MSH-(1-30)/(2-30) and pro-gamma-MSH-(1-67)/(2-67) were characterized. A third pair of peptides of still larger size was also detected. The two characterized peptide pairs terminate at sites different from the dibasic sequences typical of prohormone cleavage. This suggests either a different processing event or proteolysis during purification; in both cases cleavages are selective since the two peptides end at distinct positions. Unlike most previously described pro-gamma-MSH forms, which begin with Trp at position 1 in pro-opiomelanocortin (position -105 in relation to the start of ACTH), the novel peptide pairs show N-terminal heterogeneity with one of the components beginning with Trp as in other forms, and the second component (present in relative amounts of 10-70%) beginning with Cys at position 2 (corresponding to position -104).