In mouse spleen lymphocytes mRNA was discovered not only in poliribosomes but also in lighter RNP complexes having sedimentation coefficients of 80--120S and buyoant density of 1,40--1,52 g/cm3. The incorporation of 14C-pulse aminoacid lable has shown that the most active polypeptide synthesis takes place in light RNP-complexes and not in polyribosomes. The pattern and the contents of labeled mRNA-containing particules in spleen cytoplasm changed within different periods after the antigen injection. The greatest contents of these particles have been discovered on the second day after the antign injection, 3H- and 14C-radioactivity of RNP-complexes with higher sedimentation coefficients, and that of polyribosomes was increased; the optical density of polyribosomes peak was also increased. On the 4th day after the antigen injection contents of light RNP-complexes with higher sedimentation coefficients were decreased. On the 5th day the decrease of RNP-complexes contents was more pronounced. The changes of radioactive RNP-complexes contents in spleen cytoplasm at different periods of the immune response, which can be discovered during determined period of labling, are probably due to acceleration and then to retardation of the transition of light RNP complexes into polyribosomes. It, probably, reflects the existens in spleen lymphocytes mechanisms of translation regulation, irrespective of mRNA stability which are realized during immune reaction. Periods of the translation increasing or inhibiting in immune lymphocytes don't correspond to periods of increasing or decreasing of number of antibody synthesising cells in spleen.