The Maryland General Assembly's first ethics investigation of one of its members ended today with the mild recommendation that the House note its "disapproval" of Prince George's Del. Francis J. Santangelo's failure to fully disclose his financial interests as required by state law.

If the House follows the recommendation, as it is expected to do tomorrow, there will be no further action against the Landover Democrat. Santangelo said he will have no comment until after the House vote.

According to a report delivered to the House today by a special House investigative committee, Santangelo violated the state's financial disclosure law for a 13-month period in 1978 and 1979 by failing to report that he held an interest in firms that were winning state contracts to stage "big band" shows in a state-owned convention center.

But the committee, arguing that "the law [is] less than clear" and sympathizing with what it described as "the mental anguish and expense [Santangelo] has suffered . . . as a result of various investigations," concluded that "reprimand, censure, and all [sanctions] more severe would be inappropriate" for the 61-year-old, three-term delegate.

Instead, the committee recommended that the House "note its disapproval" of Santangelo's violation of the law "and take no further action." Committee chairman and Majority Leader Donald B. Robertson (D-Montgomery) said, "We hope and expect that will be the end of it."

The House vote would end a year of legislative investigation of charges that Santangelo improperly used his influence to help win the state contracts for the band promotion firm in 1978 and 1979. That allegation was never substantiated by the investigative committee or the legislature's Joint Ethics Committee, which asked the House to create the special investigative committee on the opening day of the legislative session last month.

Instead, both panels focused on whether Santangelo violated various disclosure requirements, either for legislators or state officials in general. The investigative report concludes that Santangelo did nothing not allowed by the House, but did not live up to the disclosure requirements of a broader law covering thousands of state officials and employes.

Robertson said the conclusion of the investigation "would demonstrate that the House is willing to take actions on ethics matters when they arise." The probe is the first under procedures that the legislature enacted in 1979 in an attempt to tighten and reform ethics laws.

The charges against Santangelo first surfaced in a Prince George's County grand jury investigation in 1979. The slim, mustachioed insurance broker was alleged to have asked several state officials and legislators to help one of his campaign workers' son, Robert Wall, win the big band contract for the 1976-1977 season at the Ocean City Convention Center from a state commission.

Wall's firm was turned down for the contract that year, but won it the following year, by which time Santangelo had obtained an interest in the promotion firm entitling him to 45 percent of its profits. During the two years firms controlled by Wall held the contract, the investigative committee found, Santangelo made $3,000.

The grand jury cleared Santangelo of criminal charges in late 1979. The legislature's investigation, which was conducted for a year by the permanent Joint Ethics Committee before the House created the special investigative committee, began in January 1980 at the request of Attorney General Stephen H. Sachs.

The investigative committee reported that the Joint Committee, which conducted most of the investigation and prepared a report in secret, found that Santangelo's efforts on Wall's behalf came at a time when he did not have a financial stake in the promotion company, and were "of the nature that a legislator might provide for a constituent.

The investigative committee agreed with this finding, and went on to conclude that Santangelo violated no House rules by voting for two bills affecting the Ocean City hall in 1977 or by failing to publically disclose his attendance at meetings at which the promotion contracts were considered by the state commission the two years Wall's company won them.

However, the report says that Santangelo should have listed Wall's firm on his financial disclosure statements for the years 1977 and 1978, because state law requires public officials to list all interests in corporations on these firms. Santangelo amended his disclosure statements in 1979 to include the promotion firm for the first time, but the report says the amendments "were not as complete as was required."