Top army officials have ignored a ruling by the senior Defense Department safety officer and have denied federal safety officials entry into two of the Army's most hazardous ammunition plants - including one that recently blew up for the third time in seven years.

Citing "a little misunderstanding," Defense Department officials said the Army paid no heed to a ruling from Assistant Secretary of Defense George Marienthal that investigators from the Occupational Safety and Health Administration be allowed inside the plant.

Last month 5,000 pounds of nitro-glycerine exploded at one of the ammunition facilities at Radford, Va., killing two workers and injuring six others. The blast followed another fatal explosion at the plant in 1971 and a third in 1974 which injured about 100 persons.

Officials of the Oil, Chemical and Atomic Workers Union filed a complaint with OSHA in January claiming that workers at another plant, in Parsons, Kan., were in "imminent danger" from hazardous working condition here.

OSHA field inspectors attempted to enter and inspect both plants on several occasions but were turned away by military or civilian contractor officials each time, Roger Clark, head of OSHA's division of field coordination, said yesterday.

Anthony Mazzocchi, vice president of the union, which represents workers at both the Radford and Parsons plants, said yesterday it was "obvious that the Defense Department's health and safety procedures are not good enough for our workers."

Clark said OSHA attempted to make its first inspection of the Parsons plant in October, and tried to inspect both plants in January after the Radford blast. "In each case we were refused by the commanding officer," Clark said. "We went to the assistant secretary of defense asking for cooperation and they ruled that we should be allowed in. Somebody down the chain of command said no."

Adm. Stuart Nelson, director of safety and occupational health policy for the Defense Department, confirmed yesterday that Marienthal had ruled that OSHA inspectors could enter plants operated by civilian contractors to the military. The Radford plant is run by the Hercules Corp., and Parsons is operated by Day and Zimmerman, official said.

"There's a little misunderstanding we're trying to get straightened out on this now," Nelson said. He said the Army was told last month of the decision to let the OSHA inspectors in.

The Army Material Development and Readiness Command (DARCOM), which is headed by Gen. John R. Guthrie, asserts that only it has the right to inspect all 14 of the Army's ammunition plants under the 1928 Ammunition and Storage Board Act, Nelson said.

He declined to comment on whether it is unusual for a military officer to ignore a ruling from the defense secretary's office.

According to military records, DARCOM inspected the Radford facility three months before January's blast. It could not be immediately learned what the inspection report showed.

Guthrie was unavailable for comment yesterday. But a DARCOM spokesman said that, despite Marienthal's ruling, the command's policy remains that "Labor has no authority to inspect working conditions in (Army contracted) ammunition plants." OSHA is part of the Labor Department.