We used an immunoperoxidase method to study the distribution of lactoferrin in seven human fetuses from 11 to 21 weeks of age. Lactoferrin was found in mononuclear cells, presumably granulocytes, in various organs from thirteen weeks of gestation onwards. In addition to the liver, spleen, lung and thymus, lactoferrin-positive mononuclear cells were also found close to vessel walls in human pancreas in fetuses more than 20 weeks old. This finding might suggest that there is significant granulopoiesis in the pancreas at this stage of maturation. In addition to the mononuclear cells, lactoferrin could be found in Hassal's corpuscles of the thymus from 13 weeks and in glandular cells of the tongue from 20 weeks onwards. The present results are in good agreement with the maturation of humoral and cellular acquired immunity and the onset of synthesis of lysozyme and interferon.