Soviet President Leonid Brezhnev is ill with a bronchial infection, Foreign Minister Andrei Gromyko disclosed today in a rare official revelation about the health of one of this country's top leaders.

Gromyko made the disclosure in explaining the unexpected absence of Brezhnev, who was 73 Wednesday, from a Kremlin luncheon he was to have hosted today for visiting Angolan President Jose Eduardo dos Santos. The official Tass agency quoted Gromyko as saying Brezhnev had directed him "to express on [Brezhnev's] behalf sincere regret that because of a catarrhal indisposition he cannot take part." Brezhnev greeted Dos Santos at an airport arrival ceremony yesterday and participated in the first round of talks as well.

The unusual Gromkyo admission, quickly disseminated by Tass and Moscow Radio, seems aimed at heading off world speculation about the health of the elderly Kremlin leadership. Premier Alexei Kosygin, the head of government and number two leader, has been sidelined since a mid-October heart attack.

The Soviets normally never break the silence cloaking the personal lives and health of the 14-member politburo, whose average age now is above 70.

In his luncheon speech, Gromyko denounced a recent NATO decision to deploy new U.S. intermediate range nuclear missiles in Western Europe. He said the Soviet Union and its allies "cannot allow" NATO to establish a position of military superiority on the continent.

"We must take care of our security and we will take care of it," he said.

Gromyko's unusual admission of Brezhnev's illness was followed by a Radio Moscow statement that the leader had "a cold."

Early last October, rumors swept the West that Brezhnev had died; the Soviets never officially responded but a few days later, he resurfaced. Brezhnev's declining health has brought endless speculation in recent years over his staying power. The Soviets, however, have had both top leaders ill simultaneously and the current situation may have spurred them to report on Brezhnev's cold.

Kosygin's heart attack and convalescence have never been referred to in the Soviet media, which furthur underlines the unusual nature of Gromyko's statement today. However, it is unlikely the Soviets will have anything more to say officially about Brezhnev's health even if his new illness confines him for a protracted period.

Andrei Kirilenko, a longtime aide, has acted for Brezhnev in the past, and the leader's closest personal aide, Politburo member Konstantin Chernenko, is thought to handle virtually all the daily paperwork for the president that does not involve major decisions.