Numerous in vitro studies have demonstrated that IGF-binding protein (IGFBP)-4 is a consistent inhibitor of IGF actions. In order to investigate the functions of IGFBP-4 in vivo, transgenic mice were generated by microinjection of a transgene, in which the murine Igfbp4 cDNA is driven by the H-2K(b) promoter, and followed by a splicing cassette and polyadenylation signal of the human beta-globin gene. Transgene mRNA was expressed ubiquitously, and elevated IGFBP-4 protein was detected in the spleen, thymus, kidney and lung of transgenic mice. The activities of serum IGFBPs were not changed in transgenic mice. Immunohistochemical studies revealed transgene expression predominantly in the thymic medulla and red pulp of the spleen. Body weight and the weights of the spleen, kidney and lung of transgenic mice were not different from controls. In contrast, the thymus of transgenic mice showed a significantly reduced weight and cortex volume. In transgenic thymus and spleen, cell proliferation was inhibited and apoptosis was stimulated. Transgenic mice showed normal T- and B-cell development and normal basal plasma immunoglobulin levels. In conclusion, overexpression of IGFBP-4 inhibits growth of the thymus. IGFBP-4 excess inhibits cell proliferation and stimulates apoptosis in lymphoid tissues, but does not affect lymphocyte development. These findings suggest that IGFBP-4 is a potential growth inhibitor of lymphoid tissues.