The lathyrogenic agents, beta-aminopropionitrile and semicarbizide, when applied to the chorio-allantoic membrane of the chick embryo produced a dramatic increase in fragility of the embryo. This alteration was not associated with a change in the concentration of collagen, except in aorta, but was accompanied by a sharp increase in the amount of collagen extractible in cold 1 M NaCl from skin, bone, and aorta. Increase in fragility and extractible collagen began within 3 hours after introduction of the agent and rose steadily for at least 72 hours. Essentially no collagen could be extracted from tissues of normal chick embryos. Both fragility and amount of extractible collagen were dosage- and time-dependent. It is concluded that the extractible collagen in lathyrism consists of a large proportion of dissolved fibers previously insoluble and formed prior to administration of the agent. The data also suggest that the "lathyritic" collagen in vivo is not in molecular dispersion but in an aggregate or fibrillar form. It is dispersed by cooling. The extracted collagen could be reconstituted to typical striated fibrils in vitro and the molecule appeared to be normal in the gross, with regard to asymmetry ratio and intramolecular helical structure. The evidence at hand suggests that at least one of the defects induced by lathyrogenic agents is an interference with the normal intermolecular cross-linking within the collagen fibril.