jp and jpmsd, two allelic mutations in the mouse that sharply reduce the amount of CNS myelin, produce diseases that can be distinguished morphologically only by their severity. This has raised the question of whether the two mutations are truly distinguishable. Since the two mutations have never been maintained on the same genetic background, correct quantitative and morphological comparison have not been possible. We have prepared a B6C3H stock of jp on the same genetic background as the available stock of jpmsd. In this jp stock, behavioral abnormalities, relative proportion of myelinated axons, and major morphological characteristics of the disease in situ are unchanged from the previous jp stock. The jp disease continues to be more severe than that of jpmsd. However, tissue from the new B6C3H stock myelinates better in organotypic culture than previous jp stocks. The increase in myelination is advantageous, not only for accurate comparison of the two alleles but for all culture studies of jp. Strictly comparable strains or stocks should be utilized in any comparative studies of closely related mutations such as jp and jpmsd.