If the drugs DDI and DDC were intended for any disease but AIDS, figuring out how well they work would be easy.

Like virtually all drugs tested in the United States, they would first be tested on a small group, to determine whether they had any serious side effects. Then, if they proved safe, they would be tried on two larger groups of patients, comparing the health of those on the drug to a group of patients given a placebo.

But the traditional way of testing drugs takes years, and most AIDS patients don't have years. Nor do doctors and researchers feel that they can ethically use placebo trials -- the quickest and most efficient way to see if a new drug works -- on AIDS patients, since that would be tantamount to condemning them to death.

As a result, DDI and DDC -- the two most promising new AIDS drugs now under evaluation by drug companies -- have become a problem for the AIDS community. While AIDS activists are eager to see the two drugs evaluated as quickly and completely as possible, government health officials, activists and researchers cannot agree on how best to do it.

Their conflict reflects, in part, how little scientists know about how the AIDS virus operates. That has made it difficult for researchers to determine how to evaluate the new drugs without a traditional full-scale trial.

In addition, AIDS activists and federal health officials disagree about how to balance the government's traditional responsibility to ensure that new drugs are safe and effective with the desperate need of those infected with the AIDS virus for drugs that work better than AZT, or zidovudine, the only drug currently approved for treating directly an infection by the human immunodeficiency virus (HIV).

How these questions are resolved, AIDS experts say, may have a far-reaching effect.

"How we decide about DDI and DDC will affect how all future AIDS drugs are evaluated," said Martin Delaney, director of Project Inform, one of the AIDS groups pushing hardest for early approval of the two drugs. "We're never going to have placebos {with AIDS}. We're never going to have long-term comparative studies. The pressure is always going to be on regulators to give us an answer very quickly."

DDI and DDC are both available under a special government program known as expanded access, which makes promising experimental drugs available to AIDS patients long before clinical testing is completed. About 15,000 patients are taking the medications under this program, but activists said that expanded access falls far short of serving the needs of all the people with AIDS.

One reason is that preliminary data have suggested that the most effective use of DDI and DDC is in combination with AZT. The expanded access program, however, prohibits using drugs in combination. It also restricts access to those who have strong adverse reactions to AZT or for whom AZT is no longer effective.

In a petition filed with the Food and Drug Administration last month, the AIDS activist group ACT-UP formally petitioned the agency to speed up its process for approving DDI and DDC. The problem, however, is that it will be at least two years before the data being collected on both drugs are complete. Although a number of trials are underway, all are either incomplete or too small to provide definitive answers about whether DDI and DDC work as well as AZT.

In addition, the drug trials are not designed to provide easy or clear-cut answers about effectiveness. For ethical reasons, none of the trials involving the two drugs includes a control group taking a placebo.

To speed approval, researchers are considering comparing preliminary data from patients on DDI or DDC to the data collected during the extensive testing of AZT. If the survival rates and number of infections for patients taking the new drugs turn out to be comparable to the results with AZT, the theory goes, the new drugs could be considered effective.

But simply comparing the results will not be easy, according to drug testing experts.

They said the strain of virus in today's AIDS patients may not be the same as the strain in the patients tested several years ago during the AZT trials. For example, many taking DDI or DDC are on the drugs only because their strain of the virus had become resistant to AZT. But some researchers suspect that whatever made the virus resistant to AZT may also have made it resistant to DDI and DDC. If so, the new drugs will appear to perform poorly even though they might be just as good when used against non-resistant strains. Even if the two drugs are as effective as AZT, they will not work as well as they might have in trials containing former AZT users.

On the other hand, in the time since AZT was evaluated, doctors have become much more sophisticated in treating AIDS patients. The most common opportunistic infection related to AIDS, for example, a lung disease known as PCP, can now be treated with steroids and prevented with the drug pentamidine. That could make it difficult to tell how much of any improvement in health on DDI or DDC is due to the superiority of those drugs and how much is due to other improvements in overall clinical care.

"Comparing an open experience with DDI or DDC to AZT, to do a historical comparison, is close to impossible because things are changing too quickly," said Douglas Richman, an AIDS researcher at the University of California at San Diego. "Every three or six months we make significant strides. Patients are doing better all the time. To attribute that benefit to any single drug is a very difficult thing to do."

Another idea is to evaluate the drugs by looking at their effect on what researchers called surrogate markers, in particular the immune system cells known as T4. One of the clearest scientific observations about HIV is that it tends to attack and kill T4 cells, which are the central players in the human immune system. Clinical evidence also suggests that keeping an HIV-infected person's T4 cell count above 200 per cubic millimeter of blood (a healthy person's count is more than five times that) correlates strongly with the prevention of full-blown AIDS and, ultimately, death.

Evaluating DDI and DDC by looking at their effect on T4 cells would allow for quicker evaluation of the drugs, some researchers said, because the FDA could use the surrogate markers as early predictors of survival rather than waiting to see how long patients survived on the drugs.

But a recent analysis of AZT data suggests that T4 cells may be an imprecise measure of the health of the human immune system. In a meeting in Boston in December, researchers said they had found that an AIDS patient on AZT with a T4 cell count of 250 is actually in better health than an AIDS patient with the same T4 cell count but who is not on AZT. In other words, fighting HIV successfully may require some extra and unknown boost to the immune system apart from maintaining a good T4 cell count.

"Just because we can't measure everything going on in the immune system doesn't mean the measure that we do have is useless," said Delaney, who contends that T4 counts are sufficient for evaluating DDI and DDC.

But FDA officials and some researchers are much more cautious in their evaluation of the usefulness of T4 counts. They said that until more is known about how the AIDS virus attacks the immune system, relying on one surrogate marker may be misleading.

One way around the problem, suggested by some researchers, might be to add other markers. When drugs for diseases such as arthritis are evaluated, data on the physicial well-being of patients, including how well they walk and how much assistance they need during the day, are collected and analyzed. These researchers suggest doing the same for AIDS patients.

"People who are taking care of AIDS patients are not simply looking at whether the patient lives or dies," said Sidney Wolfe, director of Public Citizen's Health Research Group here. "They are asking how they are doing. They are observing them. . . . There is no reason why these things can't be quantified."

Another, more sweeping solution, might be for federal regulators to concede that they may never be able to get the same kinds of answers about the safety and efficacy of AIDS drugs as they gather on therapies for other diseases where social pressures are not so demanding for improved results.

"Do we have as much data on these drugs as we would like to have? No. But I think that in this situation our standards have to be different," said David Barr, assistant director of policy at the Gay Men's Health Crisis in New York. "Not necessarily lower, but different."