The technique of heterotopic cardiac transplantation in the rat is described in detail. A simple, nontraumatic method of obtaining cellular prints from the heart is proposed for the study of cells infiltrating the allograft. Analysis of the morphological aspect of the cellular infiltrate present at the time of rejection in a series of allografts revealed a very high proportion of cells belonging to the monocyte-macrophage series, thus suggesting an important role for these cells in the rejection process.