The Securities and Exchange Commission is once again reduced to its functional minimum, after commissioner Charles L. (Lindy) Marinaccio's departure yesterday. Marinaccio, a Democrat who served 13 months on the SEC, plans to join the Washington office of the New York law firm Kelly, Drye & Warren on Sept. 1.

Republican James C. Treadway Jr. left the commission in May, and there has been no word on possible nominees for either seat. The commission now consists of Chairman John S.R. Shad, a Republican, Republican Charles C. Cox and Democrat Aulana L. Peters. In early 1984, the SEC was forced to function with only three commissioners -- the legal quorum -- for three months. LOOKING ABROAD . . . A major concern of the SEC is growing international influence on the securities markets. Eurobond offerings by American corporations -- an unregulated and therefore relatively cheap and flexible way to raise money -- have increased dramatically this year, while foreign investment in U.S. capital markets continues at high levels. The stocks of several hundred companies are traded on exchanges around the world.

As a start, the SEC has published two "concept releases" suggesting ways to improve international disclosure, surveillance and enforcement practices in Great Britain, Canada and the United States. And SEC officials have been busy stumping on the issue.

In Cartagena, Colombia, last month, Commissioner Cox stressed the need for full disclosure, American-style. Gary Lynch, director of the enforcement division, spoke at the same conference on the need for international cooperation.

Commissioner Peters, in a speech delivered partially in French to Quebec City investment dealers, said she regretted the current 25 percent limitation on foreign involvement in the Canadian securities industry. She applauded a proposal to "relax the review process for foreign firms acquiring or establishing Canadian businesses, [which would foster] a more reciprocal standard for U.S. and Canadian firms wishing to expand across our common border." BANKS AND BROKERAGES . . . Starting next Jan. 1, banks involved in brokerage services must register with the SEC. The order, which was made final July 1, has already generated opposition from the banking industry, which objects to another layer of government regulation.

More than a thousand banks now actively promote brokerage services to the public. In exchange for promotion and taking orders, the banks receive a percentage of the commission that the customer pays the broker. The SEC believes that because the services offered by the banks are virtually the same as those offered by broker dealers -- and because the banks can commit the same abuses -- the banks ought to be bound by the same rules as broker dealers.

The banking industry objects to the cost of setting up separate subsidiaries for brokerage activities, paying the administrative costs of filing with the SEC and meeting the agency's stiffer capital requirements. The American Bankers Association is threatening to file suit to stop what it considers an unnecesary and unwarranted intrusion by the SEC onto its turf. IN AND OUT. . . William McLucas, assistant director of the enforcement division, has been promoted to associate director.

Former commissioner Treadway, now in private law practice here, has been named head of the National Commission on Management Fraud, a new group that wants to fight white-collar crime, or, more specifically, the deliberate misstatement of corporate financial statements. The panel, which is financed largely by the American Institute of Certified Public Accountants, includes William M. Batten, immediate past chairman of the New York Stock Exchange, and Donald H. Trautlein, chairman of Bethlehem Steel Co.

While at the SEC, Treadway was closely involved with accounting issues. In accepting the post, he said, "Those who raise funds on the basis of false financial statements obtain capital unfairly and at the expense of honest competitors for capital."

The group will try to determine whether the current regulatory and law enforcement environment "unwittingly has tolerated and contributed" to a decline in professionalism by executives responsible for financial information.