The laboratory for development of drug forms, N.N. Blokhin Russian Cancer Research Center developed a lyophilized liposomal dosage form (LLDF) of a new infrared photosensitizer phthalocyanine derivative - Thiosens. Previous studies determined optimal composition of the dosage form, technology of its production and methods of analysis; a number of biological experiments were performed. The next step of preclinical research is studying "acute" toxicity of LLDF, which is presented in this paper. The study of "acute" toxicity of LLDF Thiosens was carried out in the laboratory of Pharmacology and Toxicology. "Acute" toxicity of LLDF Thiosens was studied in the experiments on mice hybrids F1 (CBA » C57Bl / 6J) male and female and weinbrener outbred male rats. In the first series of experiments LLDF Thiosens was administered to male mice in the dose range of 5; 10; 20 and 30 mg/kg. In the second series of experiments, male and female mice were injected with LLDF Thiosens in the dosage range 6; 12 and 24 mg/kg. Male mice were treated by LLDF Thiosens intravenously; no symptoms of toxicity were registered at the doses of 5 and 10 mg/kg; death was observed in all experimental animals after administration at the doses of 20 and 30 mg/kg on the first day in the result of inactivity and piloerection. In the second series of experiments no sex differences were found in single intravenous injection of LLDF Thiosens. The results of the studies of "acute" toxicity in rats demon strated that when LLDF Thiosens was administered in doses 3, 6 and 9 mg/kg animal deaths were reported. A single intravenous LLDF Thiosens administration in rats in the excess of the expected dose for human in 1.5-4.5 times was well tolerated by the animals: animal mortality, clinical signs of toxicity and changes in behavioral responses were absent.