Dendrimers are highly branched macromolecules with the potential in biomedical applications. Due to positively charged surfaces, several dendrimers reveal toxicity. Coating peripheral cationic groups with carbohydrate residues can reduce it. In this study, the cytotoxicity and genotoxicity of three types of 4th generation poly(propylene imine) dendrimers were investigated. Peripheral blood mononuclear cells (PBMCs) were treated with uncoated (PPI-g4) dendrimers, and dendrimers in which approximately 40% or 90% of peripheral amino groups were coated with maltotriose (PPI-g4-OS or PPI-g4-DS) at concentration of 0.05, 0.5, 5 mg/ml. Abbreviations OS and DS stand for open shell and dense shell respectively, that describes the structure of carbohydrate modified dendrimers. After 1 h of cell incubation at 37 degrees C, the MTT and comet assays were performed. PPI dendrimers demonstrated surface-modification-degree dependent toxicity, although genotoxicity of PPI-g4 and PPI-g4-OS measured by the comet assay was concentration dependent up to 0.5 mg/ml and at 5 mg/ml the amount of DNA that left comet's head decreased. Results may suggest a strong interaction between dendrimers and DNA, and furthermore, that coating PPI dendrimers by maltoriose is an efficient method to reduce their genotoxicity what opens the possibilities to use them as therapeutic agents or drug carriers. (c) 2012 Wiley Periodicals, Inc. Biopolymers 97: 642648, 2012.