Adult BDF1 and DBA/2J mice were inoculated i.p. with "L-1210" leukemia cells and then received i.p. control or immune sera raised in C27B1/6 and BALB/c mice. Undiluted sera were administered in single or multiple doses ranging from 0.2 to 0.4 ml/mouse on day 0., resp. 0., 2-4, and 7. In BDF1 hybrids permanent survivals (greater than 150 days) and distinct prolongation of the mean survival time (MST) after multiple injections were observed. However, normal serum derived from non-immunized C57B1/6 donors was also demonstrated to provide protection when injected three times. In DBA/2J recipients no permanent survivals were observed. In this mouse strain control sera proved ineffective. On the contrary, immune sera enabled the recipients to survive longer and this prolongation proved to be statistically significant (p less than 0.02). Partial natural immunity against "L-1210" leukemia in BDF1 hybrids must be, therefore, postulated. It is probably due to H-2-locus incompatibility versus "L-1210" cells, inherited from the C57B1/6 ancetor-line. Unknown factors which are present in normal serum seem to potientiate this natural resistance. In compatible DBA/2J mice normal serum constituents were ineffective, on the other hand, however, the effectiveness of specific immune factors directed against target cells of the tumor could be demonstrated in them