The human lung carcinoma cell line PG is defective in gap junctional intercellular communication (GJIC). Connexin43 (Cx43) mRNA, which is expressed in normal human lung cells, is undetectable in these tumor cells. To explore if up-regulation of Cx43 gene expression will suppress malignancy of PG cells, Cx43 cDNA was co-transfected with pSV2neo cDNA into PG cells. Control cells were transfected with the blank vector plus neo cDNA. Several stable Cx43 transfectant clones, which acquired high levels of Cx43 expression and the capacity of GJIC, were compared with control clones and the parental cell line, both of which lacked Cx43 expression and GJIC. The control clones resembled the parental cells in exhibiting high cell growth rate, weak attachment to the substratum and a high frequency of colony formation in soft agar. In contrast to the control cells, Cx43 transfected clones showed reduced growth rate, enhanced attachment to the substratum and inhibition of colony formation in soft agar. In vivo results from nude mice experiments showed high tumorigenicity with control clones and inhibition of tumorigenicity in Cx43 transfected clones. The consistency between in vitro and in vivo results strongly suggests a tumor suppressing effect of the Cx43 gene in human lung carcinoma cells.