The distribution pattern of the degree of polymerization (DP) of cellulose present in the cell walls of mesophyll- and suspension-cultured cells of tobacco was compared to that of newly synthesized (14)C-labeled cellulose from regenerating tobacco protoplasts and suspension-cultured cells. The cellulose was nitrated, and, after fractionation according to differences in solubility in acetone/water, the DP pattern of labeled or unlabeled cellulose nitrate was determined by viscosity measurements. A low (DP<500) and high DP-fraction (DP>2500) of cellulose were predominant in the cell walls of protoplasts, suspension - cultured cells, and mesophyll cells. The average DP of the high molecular weight fraction of cellulose in the cell walls of mesophyll was higher (DPβΌ4,000) than in protoplasts or suspension - cultured cells (DP 2,500-3,000). In all cell walls tested, minor amounts of cellulose molecules with a broad spectrum of a medium DP were present. Pulse - chase experiments with either protoplasts or suspension -cultured cells showed that a large proportion of the low and medium DP-cellulose are a separate class of structural components of the cellulose network. The results are discussed in relation to the organization of cellulose in the primary cell wall.