BACKGROUND: Cell transplantation is emerging as a potential means of improving repair of damaged organs. This preliminary study tests the feasibility of grafting allogenic cells into the border zone of a myocardial infarct (MI). METHODS AND RESULTS: Neonatal cardiomyocytes were obtained from fetuses of female rats 20 days pregnant. They were then injected at three different sites (2 x 10(6) cells per site) into the left ventricular (LV) myocardium of control rats (n = 10) or of rats in which MI had been created by proximal occlusion of the left coronary artery (n = 10). In the latter case, injections were placed along the peri-infarct border zone. Half of each batch of cells was grown in culture to provide a control for cell morphology and viability. Six additional rats were injected with the culture medium alone. Forty-eight hours after injection, LV slices were processed for histological (hematoxylin-eosin) and immunohistological (sarcomeric alpha-actinin transplantation and laminin staining) techniques. Examination of serial sections from injected regions showed that grafted myocytes were harbored into the host LV myocardium in all control animals and at the border zone in 50% of the infarcted rats. Grafted cells were identified by their morphological characteristics and an immunohistological pattern of loose myofibrillar organization similar to that seen in cells concomitantly grown in culture. Injection of the culture medium alone had no effect but allowed us to rule out needle-related injury. CONCLUSIONS: These initial results suggest the feasibility of transplanting allogeneic cardiomyocytes into the border zone of MI areas, a prerequisite for this approach to successfully improve the function of ischemically damaged hearts.