Have a little sympathy for Count Dracula and the Wolf Man. They were bloodthirsty, but it may not have been their fault.

A Canadian chemist reports that the film vampires and werewolves popularized by Bela Lugosi and Lon Chaney Jr. may have been based on real victims of a rare group of genetic disorders. "The folk mythology may have had some truth," said David Dolphin of the University of British Columbia. The diseases, known as porphyrias, are associated with a malfunction in the biochemical production of heme, the red pigment in blood.

Victims of the diseases are extremely sensitive to the sun and accumulate chemicals called porphyrins that can transform oxygen into a toxic substance that destroys skin tissue. The result, said Dolphin, can be devastating: Skin lesions may be "so severe that the nose and fingers may be destroyed"; as the gums degenerate, the teeth become prominent and fang-like, and "the unfortunate individuals who suffer from this disease may become very hairy."

"Imagine, if you will, the manner in which an individual in the Middle Ages would have been received if they only went out at night . . . . They would have an animal look about them, being hairy, large of tooth and badly disfigured . . . such people might well have been considered werewolves," Dolphin told a chemistry session here at the annual meeting of the American Association for the Advancement of Science.

"The story, however, goes beyond werewolves to vampires," said Dolphin, who studies the body's heme proteins.

Today, the rare and incurable disorder, carried but not always expressed by about one in 200,000 people, can be treated in part by avoiding sunlight or by receiving heme injections that can alleviate some of the symptoms, Dolphin said.

"Since in the Middle Ages an injection of the red pigment of blood would not be possible," Dolphin said, "what else might take its place? If a large amount of blood were to be drunk, then the heme in it, if it passes through the stomach wall to the blood stream, would serve the same purpose."

"It is our contention that blood-drinking vampires were in fact victims of porphyria trying to alleviate the symptoms of their dreadful disease. The folklore concerning the nocturnal behavior of vampires would, of course, be consistent with their photosensitivity," he said.

Legends about vampires suggest that their victims turn into vampires as well. "That's also explicable," Dolphin said.

Long ago, more inbreeding might have made porphyrias more common in some parts of the world. But those who carry the gene for the disease do not always show symptoms, which may be triggered by stress.

Today, attacks may be triggered by alcohol or drugs. In the past, if one porphyria victim bit another disease carrier, blood loss could have triggered it, Dolphin said. He also cited a chemical basis for the legend about sleeping with a wreath of garlic around the neck to ward off vampires.

Liver functioning involves chemicals known as cytochromes P-450 that also contain heme. They help rid the body of waste, but some drugs and chemicals can destroy liver heme functioning. A key component of garlic can do the same thing, Dolphin said.

For a porphyria victim, he said, this "could make the symptoms of the disease much worse, painful and even fatal. Can you imagine a more powerful 'talisman' against vampires than this?"

In its most severe form, porphyria can bring on nervous attacks and mental illness. Dolphin noted that King George III is thought by many to have been a victim.