Up to 80% of 32P-labeled poliovirus-RNA and RNA of other origin is adsorbed to suspended tissue culture cells (amnion, HeLa and L cells) under the conditions described. In all cellular systems the level of adsorption is highest in isotonic saline and decreases with increasing NaCl concentrations. The level of adsorption of viral RNA in physiological solutions depends upon the metabolic state of the cells and upon the heterogeneity in molecular size of the RNA. The cells adsorb 34 S-RNA better than 16 S-RNA and RNase depolymerized RNA only up to 5 per cent. No differences are found in the adsorption rate of viral RNA or RNA of other origin, provided they are of identical molecular weight. The maximum of adsorption to amnion cells is reached after 12 - 14 minutes and to HeLa cells after 20-22 minutes. Between 2 - 125 μg RNA/ml the RNA concentration does not influence the percentage of adsorption. 1 · 106 cells/ml yield adsorption values which cannot be measured exactly. Increasing the cell concentration up to 1.8 · 107 cells/ml leads to a rise in the level of adsorption. Higher cell concentrations show no further influence on the adsorption of RNA. Tissue culture cells exposed to RNA in a first adsorption cycle adsorb additional RNA in a second incubation cycle with fresh 32P-RNA, whereas unadsorbed RNA from a first adsorption cycle is poorly bound to fresh cells.