The distribution of molecules in various commercial samples of polyvinylpyrrolidone was studied by gel-exclusion chromatography on Sephadex columns. High-speed centrifugation on sucrose gradients was also used in this study. Unequivocal evidence is presented that the molecular weight spread in Plasdone C (PVP, K30) is much wider than the manufacturer's published data. A continuous distribution of molecules associated with Plasdone C from about 1,000 to 200,000 daltons was demonstrated. This material is claimed to have an average molecular weight of 40,000 daltons with a range of 20,000–80,000 daltons. Radioactive 14C PVP gave essentially similar results. Estimates based on the elution pattern of bovine serum albumin (67,000 daltons) suggest that about 50% of the molecules in Plasdone C are larger than 67,000 daltons, and dialysis experiments indicate that about 12% of the molecules are less than about 12,000 daltons. Undialyzed PVP, not toxic per se, was toxic to Pseudomonas when this bacterium was frozen and thawed in its presence. Dialyzed PVP, however, gave good protection against freeze-thaw damage. Ultraviolet absorption spectra of PVP gave maxima at approximately 203 nm unaffected by dialysis or changes in pH from 4.0 to 7.0. Standard curves are published for the estimation of pure PVP samples by either uv spectroscopy or the iodine color reaction. Dextran samples of characterized molecular weight from 10,000 to 500,000 daltons were investigated for freeze-thaw protection with rabbit erythrocytes and Pseudomonas. Dialysis had no effect on cryoactivity. Higher concentrations of dextran were better than lower concentrations in both cell systems. With erythrocytes the influence of molecular weight on cryoprotection was negligible. With bacteria, however, there was a trend toward greater cryoprotective activity with higher molecular weight.