Hope Marinden, a management analyst with the District government, didn't become a mother until she was past 40. Now she has three children, all adopted, and she also happens to be single.

What prompted Hope to plunge headlong into single parenthood, a status in some circles considered an unhappy accident?

"I didn't set out to have kids," she tries to explain. "In fact, I'd never even heard of single adoption. Motherhood simply didn't seem in the cards for me. Then, a dozen years ago, I heard about some singles adopting in California and suddenly the idea I could have kids struck me like a thunderbolt. I realized I was tired of just being somebody's aunt. I wanted children of my own."

Marinden's initial enthusiasm waivered as she "ran up against one brick wall after another." At her lowest ebb, she turned to another single adoptive mother who gave her a lead on her son Jerry, now 11. He and his brother Caleb, 9, were adopted as infants through private sources. Their sister Story, 10, a Vietnamese orphan, came through an agency.

Realizing the need for a network, Hope founded the Committee for Single Adoptive Parents, a national organization which issues a handbook and a newsletter. Its several hundred members include not only the never-married, but widows and divorcees with children who have enlarged their families through adoption. One single adoptive mother has 13 children, but two or three is more usual.

Committee members traveling to the nation's capital often end up on Marinden's doorstep.

"It's funny," she remarks, "how even when we've never met before, we discover common bonds. An adoptive mother here from California actually knew my daughter in Vietnam, even had pictures of her as a little tyke in the orphanage. So I've been able to recapture that part of her early life I thought was lost forever."

Another of Marinden's visitors from California is riding instructor Jim Forderer, who has four adopted sons, three confined to wheelchairs. Forderer, whose family travels in a specially equipped van, shrugs off any talk about his being some sort of hero or superman.

Fatherhood, he says, was his most "selfish" life decision. "I didn't adopt a cerebral palsied child who turned out to be Tommy; I adopted Tommy who happened to have C.P. No parent in the world could ever have a more delightful kid than Tommy.

"My biggest hassles," he says, "haven't been with my kids, but with agencies. Each adoption has been an adversary proceeding, with the agency trying to show why I shouldn't have a child and my having to prove myself. Only Family Builders has been helpful." (Family Builders agencies specialize in finding homes for older, handicapped and minority youngsters. Pierce-Warwick, a Family Builders agency in the Washington area, accepts single applicants of both sexes.)

Follow-up studies have exploded the myth that kids adopted by singles are "deprived." In fact, singles actually have a better adoption track record than couples, since they tend to take on children with greater problems and still manage to succeed. Compared with other single parents, this country's 2,000 single parents by adoption stand well above average in income and education.

Despite this strong showing, most singles find themselves on the defensive when approaching agencies.

"It's incredible," observes Marinden, "that with half a million American kids in foster care, there is a 'shortage' of children to adopt, with singles being relegated to the end of the line. Sometimes--even with all the redtape involved-a foreign adoption is easier, especially for a single person who wants a baby."

Virginia Clarke of Northwest Washington, a program manager at the Agency for International Development, discovered this the hard way. Though the shortage of adoptive homes for black youngsters is well known, because she was single she had no luck adopting an American child. She was successful, though, in adopting Tiffany, a black-Vietnamese child. Now a bright-eyed, talkative and endearing age 8, Tiffany came to her at 18 months--an undernourished waif from a hospital orphanage.

Says her mother, "I'm very much aware of homeless children, how many are hungry and alone in this world. I've always been especially interested in war orphans and children living on the edge of life. I just wish I had the resources to find and take in many more."

Father George Clements of Chicago, a Catholic priest who adopted a 12-year-old boy, did a lot to put single adoption on the map. But official reluctance remains.

For most singles, the key ingredient in adopting a child is perseverance. Norma Claypool, a Baltimore college professor, is a case in point. She is not only single, but also blind.

"That combination really floored my agency," she admits. "But I'm just not a quitter. I kept after them." The agency was finally won over after a social worker spent an entire day with Claypool, trying to keep up with her.

Claypool now has five kids. Two, Ricky, 4, and Elaine, 16, are also sightless. Elaine, whose birth parents consigned her to an institution as a hopeless case, now aspires "to get a job working with children in institutions. Somehow I'll fix it so every child will have a mother who wants one, no matter what the child is like."

Kenny, 8, another of Claypool's kids, has Downs Syndrome. He intuitively understands his mother's lack of sight, she says, and delights in acting as her "eyes" when out shopping. A recent family addition is Jothi, a healthy 13-year-old from India who is realizing her lifelong ambition of going to school. The newest arrival is 3-year-old Noel.

When will Claypool stop adopting? She's not willing to say. "As long as there are kids out there without families, why set a limit?"

Jim Gwaltney, 49, a single father living in the Washington area says he faced triple jeopardy.

"I was single, I was a man, and I wanted a girl. Single men, if they adopt at all, usually adopt boys. Most agencies thought I was crazy or perverted. I don't know why, but I just wanted a girl, I had to submit to a psychiatric evaluation, but I finally got my daughter."

His daughter, who was 10 at the time of adoption, is now 15. She and her dad are looking for a younger sister or possibly a younger brother and sister. But it's an uphill fight, laments Gwaltney.

"So many thousands of children and parents looking for each other, but the present system is just an impediment. Parents suffer, but the kids suffer more. I know what my daughter went through. There has to be a better way."

While most lone adoptive parents take pride in independence, they agree a back-up system is a must.

"Being on your own doesn't mean being superhuman," warns Marinden. Her own mother lives nearby and Marinden deliberately rented her basement to another single parent. "I also have several men friends my kids feel close to."

Marinden offers these additional suggestions to singles and other adoptive applicants:

* Know the law in your jurisdiction, especially regarding overseas and independent (non-agency) adoption.

* Consult other adoptive parents before approaching an agency.

* Be patient--but persistent--and eventually the door will open on a child for you.

For more information on single-parent adoption, write Committee for Single Adoptive Parents, P.O. Box 4074, Washington, D.C. 20015.