We recently reported the isolation of a novel actin-related gene, Arp11, from a PC-14 human lung adenocarcinoma clone with low metastatic and tumorigenic ability. In this paper, we report the effect of Arp11 expression on PC-14 tumor growth. We transfected a highly metastatic and tumorigenic PC-14 line, Lu-2, with an Arp11 expression vector, and obtained five stable transfectants that each express Arp11 at different levels. The tumor growth rate of these transformants in nude mice and their colony formation in soft agar were reduced in proportion to their levels of Arp11 expression. Thus, overexpression of Arp11 may suppress tumorigenicity in nude mice. The morphology of the transfectants expressing high levels of Arp11 differed clearly from that of Lu-2 and the mock-transfectants as the Arp11-transfectants were flat and fibrous, while Lu-2 and the mock-transfectants were round and were released into the culture medium. An increase in the number of actin stress fibers was observed in the Arp11-transfectants 1-12-3-x and 2-9-2 but not in the mock-transfected lines. Our results suggest that the overexpression of Arp11 regulates the transcription of multiple genes that are involved in suppressing tumorigenicity.