Incorporation of [14C]-thymidine into mouse lymph node cells stimulated with either Con-canavalin A or lipopolysaccharide in serum-free medium was markedly enhanced by the addition of transferrin. Thymidine incorporation was similar in transferrin-containing serum-free medium and in medium containing 5% foetal calf serum. Transferrins from both homologous and heterologous species were equally effective, but iron-binding half-molecules of transferrin, and low molecular weight iron chelates produced no enhancement. The optimal response was obtained with 10--50 micrograms/ml of transferrin, and with 30%--70% iron saturation. Although the major function of transferrin in lymphocyte cultures is probably to supply iron, it may also fulfil other functions.