The fate of mouse spleen cells, stimulated with Concanavalin (Con A) and labelled with [3H] thymidine in culture, has been followed after transfer into recipients, syngeneic to the cell donors. The labelled cells, localizing in the spleen, were found mainly in the white pulp in both germinal centre and periarteriolar sheath areas. The number of cells with labelled nuclei found in the spleens of recipients was much greater than could be accounted for simply on the basis of the radioactivity present. Using donor cells, whose DNA had been substituted with bromodeoxyuridine (BUdr) in sufficient amounts to increase its buoyant density, it was concluded that redistribution of label rather than cell division was responsible for this increase of labelled nuclei. In addition to these findings, the spleens of recipient mice showed evidence of a vigorous immunological response to the donor cells, very similar to that following injection of an antigen such as sheep red cells. These results show that caution must be exercised in interpreting the results of transfer of lymphocytes into recipients, even when a highly stable molecule such as DNA contains the radioactively-labelled material such as [3H] thymidine.