Reaction of white mice spleen must cells (MC) from bioprobe on toxicity of fishes from Kotokel’skoe lake during ictus of Haff disease was investigated. Results of histochemical staining shows presence of acid glycosaminoglycans, including sulphated ones, in white mice spleen MC. It is determined, that in experimental mice which were fed with fish from Kotokel’skoe lake, MC absolute number was significantly higher (in 3.7 times) compared with controls. In experimental and control mice dark MC dominate, which abundant in granules saturated with biolologically active substances. In experimental mice absolute number of MC was higher in 4.5 times compared with controls. MC functional activity in the spleen of experimental mice was also higher compared with controls, since the number of weakly, medium and strongly degranulated MC was significantly higher. In conclusion, in spleen of white mice which were fed with fish from Kotokel’skoe lake, activation of MC population occurred, which was expressed in increasing of absolute number, number of cells rich in granules and intensification of degranulation.