On the street, she is called "China Doll."

Her hair is smoothed back from her child-like face and pulled into a small knot to one side of her head. Her large brown eyes have a high exotic slant, and her lips are painted a glistening red.

China Doll is a prostitute who coolly insists that she is 19 years old. But to the detectives from the D.C. police department's juvenile prostitution unit, who found her wandering around an apartment house on Massachusetts Avenue, one night last week, she appeared young and amateurish.

They were convinced that she is one of the growing number of juveniles hidden in the world of women who prowl the city streets at night.

And it is a dismal world. There was the 11-year-old girl who D.C. police discovered worked full time for a woman who ran a house for prostitutes. There was a 10-year-old whose brother - her pimp - put her out on the street.

There were the teen-age runaways who found quick money and security in the often vicious world of pimps and prostitution.And there were the young victims, trapped or forced into the business and then explointed with threats or acts of violence.

For the last month, a D.C. Superior Court grand jury has conducted an investigation into the activities of pimps - the men who control the lives of the women on the street. The investigation was prompted by information from at least three juveniles who were involved in prostitution, sources have said.

China Doll agreed to go with the detectives to the Youth Division headquarters on Rhode Island Avenue NE, where the detectives spent two hours trying to pry out some information that would establish her name and age. Slouched in a metal chair, her hands folded over a small black purse, and her ankles crossed demurely, she wore the summer costume of the prostitute - a close-fitting, skimpy blue knit leotard and high heeled shoes. She said she was 19, but could not prove it.

She told detectives she was raised in Philadelphia by an uncle who has no telephone. She said she was staying in Washington with a family friend named "Maxine," who later told police she had nothing to do with China Doll.

She claimed a "dude" in town, a hustler, knew her, but he could not be found. She said she had never been to school although she knows how to read.

She could not remember the numbers on her Social Security card. She said she had lost the "official" identification card she bought for $6 in a shop on F Street when she arrived in town.

It was a typically tedious and frustrating session for the detectives from the juvenile prostitution unit. At 1 a.m., when they decided they could do no more with China Doll, she sauntered out of the police station and was back on the street.

The detectives, meanwhile, climbed back in their car and returned to "the strip" along 14th Street NW that is the favorite of the women who walk the streets at night.

They scanned the faces on the street corners and hunted for signs of inexperience and youth - like they saw in China Doll. And when they stopped to talk to a woman, they pushed for bits of information - like a father's name or hometown - that would help them verify a name and an age.

"We are out there to identify and remove from the street juveniles involved in prostitution," said Detective Joseph J. LeDoux, who along with Detective Gerald W. Robertson and part-time investigator. Detective John Williams make up the District police department's juvenile prostitution unit.

They are not interested in making mass arrests and hauling women to the city cellblock for soliciting for prostitution, the detectives said. They want to find the "young ladies" who are under 18 years old, the teen-age runaways and the sexually exploited children and get them off the street.

The juvenile prostitution unit had its beginnings in 1975 when LeDoux, who was working in the morals division, and Robertson, who was assigned to missing persons, decided there was a connection between prostitution and missing juveniles.

Former Police Chief Maurice J. Cullinane gave them 90 days to experiment with a juvenile prostitution unit. Within those three months, LeDoux and Robertson said, they took more than 50 youths off the street - double the number apprehended during the entire year.

In October, 1976, the juvenile prostitution unit became official. Since then, more than 200 youths have been apprehended or arrested in connection with prostitution, the investigators said.

Last week, the District police formally asked the Law Enforcement Assistance Administration for a $343,000 grant to begin a nationwide research project, centered here, on the problems of juvenile prostitution.

"There is increasing evidence that the recruitment and transport of youth for prostitution purposes is a highly organized interstate crime," the department said in its grant application. More than half of the youths involved in juvenile prostitution here are non residents, the department said, "runaways or youth abducted in other states specifically for prostitution purposes."

Once the city's juvenile prostitution unit confirms that someone they have taken from the street is a juvenile, they send them to court as a runaway or child in need of supervision" and we're done with it," said Robertson.