Italian terrorists struck for the first time today against an employe of an American company, shooting in the legs the Milan branch manager of New York's Chemical Bank.

It seemed likely that for the symbol-conscious terrorists, the choice of target was meant to be a deliberate message. For some time, the Red Bridgades have labeled U.S. multinational corporations as among their chief enemies.

Police have apparently been expecting the terrorists to turn their attention to non-Italian targets. Recent strengthened security measures include more guards at embassies here.

Two groups operating jointly claimed responsibility for the Milan shooting - the Communist Fighters Organization, Front Line, and the Communist Fighting Formations. The same groups claimed responsibility Wednesday for shooting the legs of a company executive in Milan. It was incorrectly attributed Wednesday to a terrorist group with a different name.

The Red Brigades have called on all other terrorist groups to join a stepped-up campaign against establishment authority.

Meanwhile, Premier Giulle Andreotti accepted the resignation of Interior Minister Francesco Cossiga who offered it Wednesday saying he accepted the political and moral responsibility for police failure to rescue the murdered leader Aldo Moro from his Red Brigades kidnapers. Andreotti took over the Interior Ministry portfolio himself on an interim basis.

Yesterday's leg shooting was the eighth in Milan this year and the 20th in the country. The victim, Marzio Astarita, 37, had been named to Head Chemical Bank's Milan operations in January.

Witnesses siad that he was shot by a masked man and woman as he was leaving his home for work and his wife was waving good-bye from the balcony. He was hit three times in the left leg. The assailants fled in a car.

The Red Brigades have been inveighing for some time against what they call the CIM, an acronym in Italian for "states of imperialist multinationals." They maintain that West European governments are nothing but representatives of the real power wielded by American multinational corporations.

Cossiga's resignation apparently is heading off the recriminations that seemed to be mounting against the government for not providing better protection for Moro, whose five bodyguards were killed during his kidnaping, and for failing to track down his captors during the 55 days he was held.

However, the Christian Democratic Party's leader, Benigno Zaccagnini, a close friend of the fallen leader, said: "If we have to decide on such a problem again, I think the decision will be the same."

The Communist Party hailed the resignation as "an act of honesty and responsibility." A number of Christian Democrats also applauded it. The small neo-fascist party said the rest of the government should also resign.

Local commentators said, in Cossiga's defense, that the West German police had no more luck than the Italians in their efforts to find kidnaped business leader Hans Martin Schleyer, killed by terrorists last year. The German police are widely regarded by Italians as a model of efficiency.

Police in Turin reported a success yesterday in their hunt for the Red Brigades, saying they had uncovered the Brigades' headquarters in the northern industrial city. Police said they found so much terrorist material in the two-room apartment that a complete listing would take 10 typewritten pages.

Among the items found were a list of personalities that police speculated may be a Brigades hit list, documents related to recent terrorist murders and leg-shootings in Turin, equipment to make false license plates and driver's licenses, five wig stands, Brigades literature on arms use and guerrilla warfare manuals.

Police said they found the $150-a-month apartment by patiently trying out all over the city a set of keys that they found in a car abandoned by the Brigades outside a hospital on April 11. They had driven there with one of their comrades wounded during a leg-shooting attempt that apparently failed.

A prison guard was killed after he shot back at Brigades assailants who only meant to cripple him, a Brigades statement said at the time.

The wounded Brigades member, Cristoforo Piancone, was identified by a cleaning woman as the occupant of the apartment, police said. Police said that after he was dropped off at the hospital, the Brigades people apparently hurried to the apartment to take out the most incriminating material, but did not return for the rest.