Nonionic polymers such as the poly(ethylene glycols) are very well tolerated upon injection into animals. Given the low toxicity of the polyester dendrimers in culture, we thought they would also be well tolerated upon injection into animals. However, we did not believe it warranted conducting a complete toxicity profile of the polymers at this stage of the investigation; rather, we undertook to learn if the polymers could be administered as an intravenous bolus at levels greater than 1 g/kg body weight in mice. If the polymers could be administered at this dose, a wide variety of drug delivery applications would be possible. Compound I was injected i.v. over a 10 s period into a number of animals at increasing doses. At the dose of 1.3 g/kg body weight, 1 of the 2 animals injected survived. In the animal that died, there was no evidence for blood hemolysis. The surviving animal was sacrificed at 24 h postinjection, the organs removed and visually examined. There was no gross evidence for toxicity in any organ examined.