Recent epidemiological and experimental investigations suggest a close relationship between cyclooxygenase (COX) and pathogenesis of colorectal cancer. There are two isoforms, COX-1 and COX-2, which differ in physiological functions and distribution. This study is to investigate the possible roles of both isoforms in the proliferation of colon carcinoma cells. A human colon carcinoma cell line, COLO 320DM, was transfected with an eukaryotic expression vector (pEF-BOS) carrying cDNA of either COX-1 or COX-2. Both COX-1 and COX-2-expressing cells exhibited a similar enzyme activity, 8-10 nmol/10 min/mg of protein. Growth rates of both COX-expressing cells were increased by about 2 fold as compared with mock-transfected cells. The stimulated growth of the COX-expressing cells was confirmed by the increased DNA synthesis as assessed by [3H]thymidine incorporation. Furthermore, expression of epidermal growth factor receptor (EGFR) was markedly increased in the COX-expressing cells as examined by reverse transcriptase-polymerase chain reaction (RT-PCR). A COX inhibitor, indomethacin, suppressed the stimulated growth, increased DNA synthesis and induction of epidermal growth factor receptor in the COX-1 and COX-2-transfected cells. These results suggest that not only COX-2 but COX-1 is involved in the proliferation of human colon carcinoma cells through the induction of EGFR.