In delayed skin reactions to PPD and BSA in the rat and to old tuberculin and diphtheria toxoid in the guinea-pig, intravenous injection of tritium-labelled thymidine shortly before skin testing produced labelling of a high proportion of the cells entering the perivascular infiltrates 12–18 hours later. Administration of thymidine at various times after skin testing gave immediate labelling of 5–10 per cent of cells in the infiltrates. It was concluded that the majority of the cells in the infiltrates are haematogenous, that they are synthesizing DNA before entering the lesion, and that many do so in the lesion itself. The principal cell type labelled in both experiments resembled a medium-sized lymphocyte of the lymphoid tissues or large lymphocyte of the blood. There was evidence that over several hours many of these became typical histiocytes. In non-specific reactions, a similar pattern of labelling was observed but the total number of infiltrating cells was very small.