A count of absentee ballots in Prince George's County yesterday reversed the results in two contests, apparently costing two incumbents their seats in the legislature.

Election officials stressed that the results of Tuesday's primary balloting are still unofficial and that totals are going to be routinely retallied. Once official totals are announced, candidates can demand a further recount.

After the count of absentee ballots yesterday, Del. Thomas Mooney, a Democrat, had 2,969 votes, one less than his challenger, John d'Eustachio, in a race for the 21st District Democratic nomination. When the votes cast at the polls were tallied Tuesday night Mooney had led by 21 votes.

Mooney said he has hired an attorney and plans to contest any adverse result. "We're not going to go gently into that good night," he said, paraphrasing a Dylan Thomas poem.

Mooney has served one term in the House of Delegates.

In the new 13th State Senate District, Kay Bienen, who has served two terms in the House of Delegates representing the Laurel area, first appeared to have won the Democratic nomination over her opponent, Thomas Yeager, by 41 votes. After yesterday's count, the totals stood at 3,214 to 3,236, with Bienen 22 votes behind.

Bienen said she plans to ask for a recount in her bid to move up to the State Senate.

The county Board of Election Supervisors has 10 days from last Tuesday's primaries to announce official results. The state of Maryland has another 30 days to review the results.