The effect of protein malnutrition on gingival and uterine collagen cross-linking was studied in female albino rats. The gingival and uterine samples were taken on the 28th day of the experimental period in both groups; the percent reversibility of neutral salt-soluble collagen, and the susceptibility of insoluble collagen to denaturing agents (to KSCN, urea, or pronase) were increased in gingival and uterine samples of protein-deficient animals. The analysis of gingival and uterine samples showed alpha 1 and alpha 2 subunits of neutral salt-soluble collagen appreciably increased, but beta 11 and beta 12 chains and the aldehyde content were significantly decreased in protein-deficient animals compared to controls. The results indicate that in protein deficiency the cross-linking and maturation of collagen are impaired.