SUMMARYThe distribution of51Cr-labeled lymph node cells in several syngeneic and allogeneio donor → host combinations in the rat is described. When the same51Cr-labeled cell suspension is administered to both syngeneic and allogeneic recipients, the distribution of label 24 hr later varies characteristically. (1) In syngeneic hosts, more label is usually recoverable from lymph nodes than from spleen whereas, in allogeneic hosts, more label is found in the spleen than in lymph nodes. (2) The lymph nodes of syngeneic hosts contain more label than do the lymph nodes of allogeneic hosts whereas the spleen of allogeneic hosts contains more label than does the spleen of syngeneic hosts. Syngeneic-allogeneic differences between recipients of the same cell suspension are evident in lymph nodes and spleen as early as 3 hr after injection and are highly significant at all times between 6 and 72 hr. The extent of these differences varies between different strain combinations and is not explicable purely in terms of donor → host incompatibility at the major histocompatibility locus. Within the range studied (5 X 106-2 X 108cells/rat), cell dosage influenced the proportion of labeled cells reaching the lymph nodes to a limited extent in allogeneic, but not syngeneic, transfers. Virtually no label was recovered from lymph nodes when heat-killed cells, cell debris, or supernatant from disrupted cells was injected.