Abstinence until marriage -- the A of ABC -- appears to be well understood by young people. Being faithful -- the B -- is less clear to many. The usefulness of condoms -- C -- varies widely, with some African teenagers aware that they protect against HIV infection most of the time, while others have heard mostly about their rare failures.

More important, the evidence that ABC is changing behavior is quite spotty, with condom use appearing to be the most acceptable of the three forms of advice.

ABC is a centerpiece of the Bush administration's $15 billion, five-year plan to fight AIDS in 15 target countries, most of them in Africa.

Closer to home, a study from Philadelphia found that an "abstinence only" sex-education curriculum was more likely to persuade black junior high school students to put off initiating sex than a more comprehensive curriculum.

The studies' results have something for everyone, and are not likely to end debate over whether ABC represents a balanced message of proven value or heavy-handed moralism out of touch with the reality of youth, Africa and gender inequality.

The Bush program stipulates that one-third of the money spent on preventing sexual transmission of the virus must go for "abstinence until marriage" messages.

Architects of the plan say this recognizes the usefulness of a strategy, devised by Africans, that has allowed a few countries, such as Uganda, to reduce the prevalence of AIDS infection. Critics say the Bush plan's version of ABC focuses too much on the abstinence component while reserving advice about condom use only for high-risk people, such as prostitutes, truckers and migrant laborers.

The interim evaluation of efforts in Botswana, supported by the U.S. Centers for Disease Control and Prevention, found that much of the ABC message was getting through, but that it was not making much of a difference.

A country of 1.6 million people in southern Africa, Botswana has a population-wide HIV prevalence of 17 percent. A program called Total Community Mobilization sent 450 AIDS counselors door-to-door, giving prevention advice, urging HIV testing and referring infected people to treatment. Researchers compared the people's knowledge and behavior in five districts that got this intervention with those in two that did not.

People who had talked to the counselors were twice as likely to mention abstinence and three times as likely to mention condom use when asked to describe ways to avoid infection. However, they were no more likely than the uncounseled to mention being faithful as a good strategy.

The people who had been counseled were also twice as likely to have been tested for HIV in the previous year, and to have discussed that possibility with a sex partner. However, they were just as likely to have a partner outside marriage as the people who had not gotten a visit from a counselor, and they were no more likely to be using a condom in those liaisons.