51Cr-labelled (CBA X A)F1and A strain lymph node cells were injected by either the i.v. or the i.p. route into adult A strain mice, and the distribution of radioactivity in various organs was studied over a period of 48 hours. The extent to which radioactivity in an organ is a measure of the number of living donor cells is considered in the light of control experiments with frozen-thawed and intact heat-killed cells. From these and other experiments it is concluded that the elution of51Cr and its reattachment to host cells plays little part over this period, and that the differences between the results obtained by i.v. and i.p. inoculation are largely attributable to differences in the distribution of living cells. Both with allogeneic (hybrid) and syngeneic (A strain) lymph node cells, the activity recovered in the organs was many times greater when the cells had been administered by the i.v., as opposed to the i.p., route. This was true for the liver, lungs, spleen, and lymph nodes, but not for the thymus and salivary glands, from which little activity was recovered by either route. Using both routes the greatest part of the recovered radioactivity was found in the liver and spleen, but relative to organ weight uptake was highest in the lymph nodes and spleen. It is suggested that the different distribution of cells, together with the fact that cells injected i.p. are exposed to peritoneal as well as to liver macrophages, accounts for the very different results obtained when attempting to induce sensitization or tolerance by these two routes. Adult A strain mice presensitized with CBA antigens showed what appeared to be immune elimination of hybrid lymph node cells—an observation suggesting that it might be possible to use this approach as a test of homograft sensitivity.