Cardiac specialists at Johns Hopkins Hospital announced yesterday that they are now ready to transplant a human heart and lungs, making the hospital the fourth such center in the United States.

But the new Heart and Heart-Lung Transplant Service is suffering the same problem facing other medical centers involved in transplants--a lack of body organs. Although the development of an experimental drug, cyclosporine, encourages transplants by reducing the rate of organ rejection, the lack of organ donors presents a larger barrier.

Two candidates for transplants at Johns Hopkins, both men in their 20s from out of state, have been selected and have been waiting from one to three weeks for organs to be donated.

One man is seeking a new heart, the other a heart and lungs. The hospital would not identify the candidates, one of whom lives in a community that is trying to raise money to pay for his operation. One candidate remains in Johns Hopkins Hospital, the other is living in a Baltimore apartment while waiting for a donor.

The Johns Hopkins transplant center became possible after the university last year hired Dr. Bruce Reitz, who developed the technique for combined heart-lung transplants and performed the first six attempted in the U.S., and Dr. William Baumgartner, both of Stanford University Medical Center. Before coming to Baltimore, the two surgeons performed almost 100 heart transplants.

Fewer than 30 patients worldwide have received a new heart and lungs "package," Reitz said, and of those, 13 are living. Four of the six patients Reitz operated on are now living.

The doctors said that at present, the surgery would be restricted to those under 50 who have such ailments as severe coronary artery disease or birth defects in the heart muscle. The patients cannot be younger than 13 or 14, the doctors said, because they must take drugs that suppress the immune system and that also stunt growth. A patient must either speak English, or have an interpreter, be psychologically healthy and have the resources to pay for the care, the doctors said.

Reitz said the operation will cost between $40,000 and $50,000. Reitz said this is less than the $62,000 charged at Stanford because of their experience in the procedures and because medical costs are lower here than in California.

The other American hospitals performing heart-lung transplants include Stanford, Pittsburgh Presbyterian Hospital, and the Texas Heart Institute in Houston.

At present, the hospital is dealing only with candidates who can pay the full amount, or whose insurance will cover it. The federal government's Medicare program gave up paying for the costly operation after a few initial cases. Baumgartner said the hospital had not yet asked Maryland's Medicaid program whether it would cover the heart-lungs transplants, which the federal government still considers experimental, or the heart transplants.

Private health insurers "will cover the transplants under major medical," said Reitz. "They don't print this in their policy, but on a case-by-case basis, they'll deal with their policyholders."

No approval by the state health planning agency was needed to introduce the transplant center, said Ann Jones, a hospital vice president.

To speed the organ donation process, three Baltimore area companies are making their corporate aircraft available to the hospital to retrieve organs or bodies, said Steve Push, a hospital spokesman. The Baltimore city and county police and fire departments, as well as the Maryland State Police helicopter unit have promised their cooperation as well.

All donors to the program must be under 35, Baumgartner said, because those over 35 have a greater tendency to develop coronary artery disease.