1) The ability of choleragen preparations to catalyze the activation of adenylate cyclase, the ADP-ribosylation of arginine and protein, and the hydrolysis of NAD is an intrinsic activity of the A1 peptide. Choleragen is thus capable of activating the ribosyl-nicotinamide of NAD in the absence of the physiological acceptor and is similar to E. coli heat-labile enterotoxin, Pseudomonas Exotoxin A, and diphtheria toxin. 2) Differentiation of the 3T3-L1 preadipocyte is associated with loss in ganglioside content, a decrease in the choleragen receptor, ganglioside GM1, and a reduction in surface GM1 as estimated by choleragen binding or fluorescent staining of bound choleragen. GM1 thus serves as a marker for the transition from fibroblast to adipocytes. 3) An ADP-ribosyltransferase, which catalyzes the stereospecific ADP-ribosylation of arginine and protein and, to a lesser extent, the hydrolysis of NAD, was identified in avian erythrocytes and purified greater than 500,000-fold with an 18 percent yield. One protein band of 28,300 molecular weight was observed on sodium dodecyl sulfate-polyacrylamide gels. Activity cochromatographed with protein on gel permeation columns. The turnover number of the purified protein was 9,000 moles min-1 enzyme-1.