When Marjorie Guthrie phones a senator, she first gives her name to the administrative assistant. She gets the standard line - "He's in a meeting." Then she asks the assistant's age. Why do you want that? is the reply. "Well," she explains, "if you're under 30, I'm Marjorie Guthrie who is Arlo's mother, and if you're over 30, I'm Marjorie Guthrie who is Woody's wife."

That gets her through to the senator.

From there it's a snap. She shares her latest thinking about Huntington's disease, the neurological disorder that took her husband's life in 1967. Woody Guthrie - a giant in American folksinging, the author of "This Land Is Your Land," a befriender of the poor and the exploited - spent most of his last 15 years in hospitals suffering a slow, excruciating death. Much in the style by which Woody had aligned himself with forgotten causes and became what he called "a world hoper," the hope of Marjorie Guthrie is to stir public awareness about Huntington's disease.

As a living memorial to a national folk poet, that would be noteworthy enough. But in a decade of work, she has become a canny citizen activist. She has her own organization in New York - the Committee to Combat Huntington's Disease. Perhaps more crucial, she knows that having a worthy cause means little unless you can master the methods of advancing it in political and bureaucratic Washington.

That she has done. Armed with facts, research and the charming bounce of the Jewish grandmother that she is (Arlo has three children), she badgered Congress until in 1976 it created the Commission for the Control of Huntington's Disease and Its Consequences. She was appointed its chairwoman.

She was not so naive as to think that the formation of a commission would be anything more than a muscle flex in preparing for the hardwork ahead and continued fighting on a number of fronts. When she made the rounds in Congress, one reaction was that Huntington's is just another disease of the month - and next month it will be something else.

Like a sick cell, this attitude can spread. The pharmaceutical industry, for example, finds little economic incentive in developing new drugs for the comparatively small Huntington's market. The disease affects about 1 in 10,000. Big profits aren't to be found here, even though Huntington's and similar neurological diseases are a major portion of the national morbidity and mortality rate. Thus, as large drug companies avoid research into this and other central nervous system disorders, tens of thousands suffer and die.

It isn't much brighter in the federal health establishment. As part of the National Institutes of Health, the National Institute of Neurological and Communicative Disorders and Stroke has a few officials making the good fight for Huntington's. But the competition for available money is fierce. The report of Guthrie's commission spells it out: The neurological institute gets only one-sixteenth of all NIH funds, and out of that share less than 1 percent goes to research for Huntington's.

As research into the neurosciences sinks to what one official calls "disastrously low levels," Marjorie Guthrie keeps prodding the health establishment. This month, she is lecturing at Harvard's medical school, MIT and the University of Kentucky. "When I'm among physicians," she says, "I don't talk science. I talk human beings. I try to persuade them that their work is not just with cells or sophisticated medical machines. They're working on people."

She herself worked with Woody all through his devastating illness. Many doctors wanted to leave him as a vegetable. Others misdiagnosed him as alcoholic or mentally deranged. It is understandable that only brave doctors go near Huntington's. Few disease are as mercilessly ravaging to both the body and mind. No cure is known. It has no remission and can last for 10 to 20 years. Death can be the least of the agonies.

When they met in 1942, Marjorie Greenblatt and Woody Guthrie were an unlikely match: he, a self-educated and footloose radical from the Oklahoma dust bowl, and she, a dancer from the Martha Graham troupe. Yet, what Woody was doing all those years - jolting the system, singing out that one man can make a difference, taking on the lost cause - is what Marjorie does now.This lovely woman makes a music of her own.