The vitamin B-6 antimetabolite 4-deoxypyridoxine, when injected into 13-day chick embryos, has the effect of increasing the amount of collagen solubilized from the leg bones by buffered saline solutions, 24 h after the injection. This effect is similar to, but less marked than, that produced by the administration of the lathyrogen beta-amino-propionitrile. Since that fraction of the total collagen which is solubilized by saline represents the least-cross-linked pool, it is concluded that 4-deoxypyridoxine is a lathyrogen, decreasing the cross-linking in the developing embryo, and confirming the importance of vitamin B-6 in that process. Lysyl oxidase, the cross-linking enzyme, was measured in extracts made from the epiphysial cartilages of embryos 24 h after the injection of either 4-deoxypyridoxine or beta-aminopropionitrile. The injection of 5 mg of beta-aminopropionitrile causes the lysyl oxidase activity to fall to 61% of that of saline-injected controls; after treatment with 4-deoxypyridoxine, the activity is 74% of the control value. In the latter case, full activity is not restored to the extracts by preincubation with pyridoxal phosphate. The results are discussed in relation to the early development of the connective tissues.