Lactoferrin has been identified as a factor in human colostrum that accounts for increased incorporation of thymidine into the DNA in an in vitro rat crypt enterocyte bioassay. We have examined lactoferrin-stimulated thymidine incorporation by comparing the effects of iron-free lactoferrin (apolactoferrin) with those of iron-saturated lactoferrin (diferric lactoferrin) under conditions that inhibit the transfer of iron between these iron-binding proteins in the bioassay system. In addition, we have compared the dose-response relationships of diferric lactoferrin and apolactoferrin. The results demonstrated that lactoferrin, independent of iron-binding states, promoted the incorporation of thymidine into the DNA of rat crypt enterocytes. These observations suggest a previously unreported nutritional role for lactoferrin that is independent of its iron-binding capacity.