Heart transplantation is the most effective therapy for chronic severe heart failure, but there is an extreme shortage of hearts available. We examined the possibility that cardiomyocytes can be modified genetically prior to being grafted to the heart. We used a replication-defective retrovirus carrying the beta-galactosidase (beta-gal) reporter gene. The beta-gal gene was transduced into murine fetal cardiac myocytes by culturing a recombinant retrovirus-producing cell line in a Transwell plate hung into the primary cardiomyocyte culture. The cultured cells were stained with the di-beta-D-galactopyranoside (FDG) and were sorted by fluorescence-activated cell sorting (FACS). FACS analysis showed that 25.5 +/- 4.3% of the cardiomyocytes in a primary culture were positive for beta-gal activity. These cells were transplanted into the hearts of syngeneic adult mice. Expression of the beta-gal gene in the grafted cells was demonstrated by staining with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside (X-gal). Gene expression was recognized as long as 6 mo after cell transplantation. Histologic analysis showed neither inflammation nor fibrous scar tissue on the host myocardium. This study demonstrated that genetically modified cardiac cells were transplantable to the heart.