Three years after Harry Sullivan retired from the Army he received a familiar greeting in the mail from the Defense Department. Uncle Sam still wanted him.

Sullivan, 51, who retired in 1978 after serving 24 years, was told that he should be prepared to report for active duty at Fort Belvoir within seven days after the president declared a national emergency or the secretary of the Army began mobilizing troops.

Sullivan also was told that his orders were valid until he reached his 60th birthday unless he became physically incapacitated.

Since last November, the Army has mailed similar notices, known as "hip-pocket orders" because they go into effect only under special circumstances, to 98,000 retirees as part of a little-publicized retiree mobilization program.

The program has been so low-key, in fact, that the General Accounting Office recently reported that commanders at several military installations, including the Military District of Washington, did not realize the program existed. Nor had they made any plans to deal with several thousand retirees who had been ordered to report to their bases during an emergency.

The GAO said it doubted whether the plan would work, mainly because the Army has not determined whether those who received the notices actually will show up. During any given year, the GAO said, 9,000 of the retirees would be hospitalized for one or more days, 3,000 would have become disabled and 2,300 would have died -- factors the Army had not considered.

The GAO expressed several other doubts about the retiree mobilization plan, including whether the program is necessary during a time when all of the services have met their recruitment goals and, in some areas, have drawn up waiting lists.

But, the Army contends that its plan is vital. Current recruitment needs are based on peacetime conditions, said Lt. Col. Ed Thomas, a manpower expert at the Pentagon. If there were a war, the Army estimates that it would need an additional 179,000 trained soldiers. By recalling retirees, the Army would gain a force of already trained professionals who could free younger troops for combat, Thomas said.

The Army first considered recalling retirees after the draft ended in 1973, but it did not develop a mobilization plan for older soldiers until 1980. It sent out mandatory orders last year, and opened the plan to volunteers in July, a Pentagon spokesman said. All of the services are developing similar programs, but the Army has sent out the most notices, the spokesman said.

The Army began its program by reviewing the 239,000 retirees under 60 who were listed as being in good physical condition. It then compiled a list of more than 180,000 jobs that it felt older soldiers could perform.

The center then matched the list of jobs with the list of names and settled on 96,700 retirees, who then received their pre-assignment notices. This summer the Army notified several thousand other retirees that they could volunteer for the program, and 1,300 did.

The GAO said its review showed that few retirees objected to the orders, because they saw them as part of a continuing responsibility to their nation. "I feel it is a real privilege to continue serving if our country needs us," said Sullivan.

Despite that enthusiasm, the GAO questioned whether all of the retirees' could do their assignments satisfactorily, especially since the Army now has a number of weapons that it didn't when they were in service.

James N. Juliana, a deputy assistant secretary of defense, said the GAO report was unfair because it was done while the program was being implemented. He admitted that the plan had several problems, but said they would be resolved. For instance, he said, the Army will study whether to offer refresher courses for retirees.