Lymphocyte infiltration of heart grafts has been monitored using pulses of Indium-111-labelled syngeneic lymphocytes. The cells were injected into cyclosporin (CSA)-treated or untreated rats that had received an allograft 1, 2, 4, 6, and 8 days previously Accumulation of the labelled cells in the graft was measured 24 hr after injection, and was compared with that in the animal's own heart. For the first three days after grafting, Indium-111-labelled lymphocytes accumulated to the same extent in the grafts of untreated and CSA-treated rats. However, the substantial rapid increase in lymphocyte accumulation in the graft that occurs in the untreated recipient between days 4 and 5 did not occur in CSA-treated recipients. From day 5 until day 9 the accumulation of labelled cells in CSA-maintained grafts was not greater than in syngeneic non-rejecting grafts, and it was significantly less than in the untreated rejecting grafts. At early times after grafting, removal from the labelled cell population of lymphocytes with reactivity against the histocompatibility antigens of the graft resulted in a reduction in the extent to which these lymphocytes accumulated in the graft.