Keinyo White was 2 years old when the first, tell-tale patch of pale white skin appeared at one corner of his mouth, shocking in its contrast with the rich brown of his normal skin.

"we started taking him to dermaltologists, and they didn't know what it was," said Ruby White, the 9-year-old's mother. "One said Keinyo was using too much toothpaste, or chewing too much gum," she said, shaking her head incredulously.

The changes in Keinyo's skin color were caused, the Whites later learned, by vitiligo, a little understood condition that affects between 1 and 3 percent of the population.

Vitiligo is "a disease of the pigment cells," said Dr. John A. Kenny Jr., chief of dermatology at Howard University Hospital and codirector of a new $2 million, federally financed, study of the condition that will take place at five colleges.

"It doesn't kill, as far as we know, but there are severe psychological effects," said Kenny, who treats Keinyo White and about 150 other vitiligo victims, most of them black.

The condition strikes blacks and whites in equal numbers, but because it turns patches of skin white, it is far more noticeable in blacks. While whites with the disease have to be careful about going out in the sun, because a sun tan makes the condition more visible, the disease can be a constant source of embarrassment for its black victims.

William H. Tate, a 65-year-old retired meat cutter, first noticed a single white spot on his hand when he awoke one morning in 1946. "There was just one spot. I don't remember which hand now, "said Tate, whose hands are now white with a scattering of dark brown spots where the condition is responding to treatment.

"It just came overnight. First one spot and then another. I was quite sensitive about it, "said Tate, who has white pathces all over his body, including some around his mouth. "I was always thinking people were looking at me. Young children would say, 'Look at him! He's half black and half white!'

"I wore long-sleeve shirts for a long time, "said Tate. "I wouldn't put on my short-sleeve shirts in the summer time. But then I saw other people with it and I figured, what the heck!"

When Keinyo White is asked about the white pathces of skin on his back and an upper arm -- the patch near his mouth has responded to treatment -- he tells people, "I was popped by bacon."

As Keinyo gets older he is becoming more sensitive about his condition, Ruby White said.

"We're just beginning to notice it. We took him on vacation this summer and he was sensitive about taking off his shirt. We tell him he's a beautiful little boy and has a beautiful body," said White. "We want him to be able to handle [the situation] if the children begin teasing him about it."

The condition can appear suddenly, or slowly over a long period of time. It may be limited to a small, single area of the body, or may cover great patches of skin in different areas. And there is no way of knowing when it first appears how large an area it will affect or whether it will respond to treatment.

Tate said he has been under treatment for about 18 months, and Keinyo White has been under Kenny's care for almost six years. Both are making progress but both still have vitiligo.

The condition does not appear to be genetic, said Kenny, although 30 percent of the cases run in families. Whatever the disease is, he said, it appears to attack the melanocytes, the pigment-producing groups of cells between the upper and lower skin layers.

The federal study, which is being conducted at Howard, Yale, Harvard, Bryn Mawr, and the University of Pennsylvania, is seeking to find the cause of the disease, improve treatment methods and examine its psychological and social effects on its victims.

"We can treat vitiligo, and some patients respond very well," said Kenny. "Others, we can give treatment and they don't respond at all."

Patients are treated with a drug called Psoralen, developed in Egypt in the 1940s, said Kenny. The drug is derived from a plant root used by the ancient Egyptians to treat what may have been the same condition, he said.

After taking the drug orally, or having it painted on the skin, the patient is exposed to either sunlight or ultraviolet light in the large "light box." The treatment may be as brief as a second or two, or may last a half hour, said Kenny, depending upon the sensitivity of the patient's skin.

One of the goals of the Howard research program, said Kenny, is to develop methods for measuring the amount of the drug that is absorbed into the patient's bloodstream.

It may be, he said, that patients who do not respond to treatment do not absorb the drug at all.