This work aims to evaluate biological effects of naproxen through of the labeling of blood constituents with technetium-99m (99mTc), survival of bacterial cultures and electrophoresis profile of plasmid DNA. Blood samples from Wistar rats were incubated with naproxen or with saline (0.9% NaCl) as control, and radiolabeling of blood constituents was performed. Influence of naproxen on the E. coli AB1157 culture growth and survival in the presence or absence of SnCl2 was used to assess cytotoxic effect. Electrophoretic profile in agarose gels of bacterial plasmids in presence or absence of SnCl2 was used to evaluate antioxidant and genotoxic potential of naproxen. Results obtained suggest that naproxen could not interfere on the labeling of blood constituents with 99mTc, but it could present cytotoxic effects and protect E. coli cultures from the lethal effect of stannous chloride at low concentrations. Moreover, naproxen could present genotoxic effect in isolated plasmid DNA.