Doctors say they fused a natural human protein with diphtheria poison to create a medicine that can dramatically relieve the pain and swelling of severe rheumatoid arthritis.

"It's really remarkable pain control. One lady went dancing. One woman said she felt like Cinderella," said Dr. K. Lea Sewell.Testing is still in the preliminary stages, but doctors said virtually all symptoms of the disabling disease were eliminated in about one-quarter of those who had the treatment.

The treatment, interleukin-2 fusion toxin, is produced by Seragen Inc. of Hopkinton. It is being tested elsewhere for use against some forms of blood cancer.

Dr. David Trentham, who also worked on the study, said the treatment has the most potential of any arthritis drug developed since the benefits of methotrexate were recognized about five years ago.

In testing at Boston's Beth Israel Hospital, 13 patients with severe arthritis took injections of the new drug daily for a week. Four showed dramatic improvement, while eight others had at least some response.

"Three of them felt like they did not have arthritis," Sewell said.

"I think this agent has a lot of potential," commented Dr. C. Frederick LeMaistre of the University of Texas at San Antonio.