Peanut (Arachis hypogaea L. subsp. fastigiata var. vulgaris) developing seed microsomal polypeptides of the high oleic acid variant (F435) and its presumed isogenic, normal line (78-1339) were compared using two-dimensional gel electrophoresis. A pair of 20 kDa polypeptides focusing at pH 6.8 and 7.3 was present in all of the polypeptide profiles from both isolines regardless of maturity or genotype except for (F435) at stage 1 maturity. The stage 1 (F435) profile contained, instead, an 18 kDa polypeptide pair focusing at about pH 9.3. Based on correlation evidence, we postulate that the 20 kDa polypeptides could be components of the $DT¹²-desaturase complex. The 18 kDa polypeptides appeared to be associated with lower desaturase activity, reduced linoleic acid and increased oleic acid seed content. Since the 18 kDa polypeptides focusing at pH 9.3 are not found at later stages, they are probably under developmental control. Changes in the developing seed polypeptides of the microsomal fraction over the four maturities are also reported.