WHILE MARYLAND IS STILL absorbing the impact of the Marvin Mandel conviction, another federal investigation involving top public officeholders, friendships and cash is unfolding in the District of Columbia. There's no telling where this particular effort will lead, but prosecutors have been examining, among other things, a most intriguing relationship between two prominent local figures. One is Joseph P. Yeldell, the man who parlayed an abominable record as director of the city's Department of Human Resources into a job as No. 3 man in the Walter Washington administration. The other is millionaire developer Dominic Antonelli, who provided Mr. Yeldell with some impressive personal financial assistance - and who benefited from some handsome business arrangements with the city government.

According to a report by staff writers Karlyn Barker and Milton Coleman in this newspaper, witnesses in the investigation say that it has been broadened to include an examination of allegations that a secret cash find was used to pay some expenses in Mayor Washington's 1974 campaign. The prosecutors apparently are interested in the origin of this alleged fund and what role, if any, the mayor's No. 2 man - Julian R. Dugas - may have had in the campaign-financing arrangements. Both Mayor Washington and Mr. Dugas have denied knowing anything about a secret fund of any unreported payments to campaign workers.

Without prejudging any of the legal aspects under investigation, the relationship between Mr. Yeldell and Mr. Antonelli would seem to be precisely the sort that prudent government officials should take pains to avoid. In this instance, it began when Mr. Antonelli contributed to Mr. Yeldell's political campaign to be delegate to the House. Next, Antonelli arranged - through a bank in which he is a major stockholder - a loan for a private business Mr. Yeldell ran on the side (it's now defunct).Mr. Yeldell subsequently did the following things:

Urged the city to sign a 20-year, $5.6 million lease on an office building that Mr. Antonelli had just purchased for $800,000.

Recommended a shift in the site of a hospital in which Mr. Antonelli had a financial interest to a property owned by Mr. Antonelli; and

Sought city funds to buy a downtown hotel in which Mr. Antonelli also had a financial interest.

Last year, Mr. Yeldell again turned to Mr. Antonelli for help in arranging a $33,000 personal loan. This was provided by Mr. Antonelli through an intermediary, according to federal prosecutors - shortly before Mr. Yeldell asked Mayor Washington to approve the leasing of that Antonelli building.

We do not assert that there is a connection through all this of a sort that would constitute a violation of law. That is for the authorities and the judicial process to determine. For now, we would only note that Mr. Yeldell - by receiving personal financial assistance of this magnitude from someone for whom his city government agency was in a position to do favors of a profitable nature - again showed a remarkable insensitivity to the public responsiblity entrusted to him. For his part, Mayor Washington has consistently failed to recognize this fundamental weakness in his administration. That is why the federal investigation should be pursued vigorously - for the sake of the city officials concerned as well as the people whom they are sworn to serve.