We have shown that the developmentally regulated appearance of the two myelin-associated glycoprotein (MAG) polypeptides in normal mouse brain myelin does not reflect the developmental pattern of differential splicing of primary gene transcripts as determined earlier by RNase protection assays. Contrary to expectation, the large (L-MAG) and small (S-MAG) polypeptides are present in about equal amounts at a relatively early stage of myelination, day 24 or earlier. In quaking (qk) mutant mouse brain myelin, both MAG polypeptides are evident at all ages examined; the relatively greater abundance of S-MAG compared to L-MAG at early ages (days 18 and 22) confirms our earlier observation on in vitro translations of mRNA. At later ages (day 27 and beyond) both isoform are present in approximately equal amounts. The L-MAG but not the S-MAG polypeptide can be phosphorylated by kinases that are endogenous to isolated qk myelin, analogous to the phosphorylation we have observed in vivo in normal mice and in their isolated myelin.