The lathyrogenic effect of INAH in the chick embryo may be measured by the increase in the extractibility of collagen from the bones with 1 M NaCl. Incubation of these bones in vitro with carbonyl compounds diminishes the amount of extractible collagen; with D-L-glyceraldehyde the reversal of the INAH effect is complete. This reversal effect is dependent on the time and temperature of incubation and on the quantity of D-L-glyceraldehyde, but is independent of the pH of the incubating medium, the optical form of the glyceraldehyde, or the metabolism of the cells; this suggests that it depends on a simple chemical combination. D-L-glyceraldehyde also reverses completely the extractibility of collagen from the bones of embryos rendered lathyrogenic with BAPN, semicarbazide, and hydrazine hydrate. The hypothesis has been advanced "that lathyrogenic agents act by blocking carbonyl groups on the collagen molecule, thus preventing cross-linking essential to normal maturation; normal maturation may be restored by the addition of carbonyl groups which act by competing either for the lathyrogen or for functional sites on the collagen molecule." In support of this hypothesis, it has been shown that purified lathyritic guinea pig collagen takes up lesser amounts of 2,4-dinitrophenylhydrazine-a compound which combines with carbonyl groups-than does normal collagen; it has been shown that lathyritic collagen still possesses the ability to form segment-long-spacing (SLS) collagen, but that these fibres are much thinner than normal; this is due perhaps to blockade of groups essential for lateral cross-linking of the tropocollagen unit. It has also been shown that normal, purified guinea pig collagen which has been pretreated with INAH, takes up lesser amounts of 2,4-dinitrophenylhydrazine and forms much thinner SLS fibres than the untreated controls.