In spite of the considerable progress made in recent years toward the understanding of the chemistry and biological function of the cobalt-containing B(12) group of compounds, much of the information still is more descriptive than definitive in nature. In general terms, it is known that the free vitamin forms can function as methyl group carriers and that the 5'-deoxyadenosyl or coenzyme forms serve as hydrogen carriers; but the mechanism of these processes is not understood in detail. More systematic studies of the pure chemistry of these complex molecules containing a carbon-cobalt covalent bond are needed before the biochemist can interpret many of his observations on the enzyme-catalyzed reactions. Even in relatively simple solutions it is difficult to ascertain the state of oxidation of several of the vitamin forms, and these problems are compounded when the reactive thiol compounds and complex proteins of the biological systems also are present. For example, both vitamin B(12r) (the Co(2+) form) and corresponding analogs are known to disproportionate in solution to B(12s) (Co(1+)) and B(12a) (Co(3+)) under a variety of mild conditions (12, 57). This means that in the biological systems it is exceedingly difficult to ascertain the chemical nature of many B(12) intermediates and reaction products. The role of the protein moiety of the various B(12)-linked enzymes in the catalytic processes is little known as is, also, the mode of binding of the B(12) derivative to the protein. These types of questions perhaps can be answered eventually by the crystallographers, whose art is becoming increasingly sophisticated. Note added after preparation of manuscript. In contrast to the values given in Table 4 for the molecular weights of the two dissimilar protein moieties of glycerol dehydrase, a recent report (57a), gives a value of 188,000 for the molecular weight of a stable, catalytically inactive complex of 1 mole of hydroxocobalamin and 1 mole of the apoenzyme complex of glycerol dehydrase. The latter is presumed to contain one equivalent of each of the two dissimilar protein subunits. The original estimate of 240,000 as the molecular weight of the unstable sulfhydryl protein moiety (39) was undoubtedly made on partially aggregated material.