Through an analysis of the immune response of mice to sheep's erythrocytes within a system of syngenic transfer the action of Methotrexate on the T- and B-lymphocytes was studied. Splenic cells of donors receiving methotrexate were introduced to lethally irradiated recipients together with the bone morrow or thymus cells. In the recipients' spleen the content of antibody-forming cells and hemolytic foci were determined. It was shown that in recipients receiving splenic cells alone the number of antibody-forming cells and of the foci sharply declined under the effect of methotrexate. Introduction of the bone marrow cells contributed to reconstitution of the response in recipients at the level of the antibody-forming cells, but not at that of the hemolytic foci. At the same time, thymocytes increased the content of the antibody-forming cells up to a level exceeding the control one, as well as the number of foci. The results proof methotrexate capable of inhibiting the proliferation of the T- and B-cells in the spleen of mice, induced by the antigen.

The content of direct plaque-forming cells (PFC) in the spleen of sheep erythrocytes immunized mice after a single administration to them of 2.5 mg/kg methotrexate on the 48th or 72nd hour of the immune response was measured. The PFC count was determined every 4 hours for 68 hours after introduction of methotrexate. In either of the cases the curve reflecting changes of the PFC count in the spleen was of an undulating nature. The alternating rises and falls of the PFC number proceeded at a high speed, viz. the time of doubling and half-periods of drop-off on individual lengths of the obtained curves was of the order of 2--4 hours. It is suggested that the PFC population accretion is ensured by a synchronous recruitment from the cells-precursors, antigen activated upon introduction of methotrexate. The decline in the number of PFC is, apparently, dependent upon their destruction due to an "unbalanced growth".