A man in remote modern North America finds evidence for an undiscovered group of American natives still living in the area. He enlists the help of a female anthropologist who works in the region and speaks a local Indian language. They do indeed find the natives and manage to befriend them. The chief's son has a bad fever and could be saved with modern antibiotics. Our hero rides back to town to obtain antibiotics without prescription. Having aroused suspicion, he is followed back to the natives' camp and their isolation is broken for good. As far as I recall, it all ends tragically (showing once again that white man never did any good for the natives) but I'm not sure anymore how it really ended. Any thoughts ?

I believe it was called something like Last of the Dog Men. With Tom Berenger, if I recall correctly.