The influence of increasing concentration of the highly polymerized dectran Ficoll on cultured peripheral human blood lymphocytes stimulated or not stimulated by PHA was studied. The incorporation of 14C-thymidine into acid-insoluble material of unstimulated lymphocytes has not been influenced in the presence of increasing concentrations of Ficoll either pretreated with chelating resin or not pretreated. Ficoll not pretreated with chelating resin potentiates the PHA-induced stimulation by the factor 4.3 at 0.1 mg Ficoll/ml culture medium, and by the factor 3.2 at 1.0 mg/ml when PHA stimulated HPBL were used. Ficoll after pretreatment with chelating resin does not influence the DNA-synthesis at 0.1 mg Ficoll/ml medium, but causes a drop of the incorporation of 14C-thymidine by the factor 2.0 at 1.0 mg/ml. Using 10.0 mg per ml, both Ficoll preparation cause a decrease of the DNA-synthesis by the factor 2.5--3.0--probably a cytotoxic effect. The results obtained with emission spectrographic analysis and conductivity measurements show, that Ficoll after treatment with chelating resin contains about 10 times more ions (Mg, Ca, Na, Si) and has a 3.2-fold higher conductivity then Ficoll without pretreatment. It is possible that phenomena like electrochemical changes on the surface of the lymphocytes and osmotic alterations in the culture medium are responsible for these effects.