Fairfax County prosecutor Robert F. Horan Jr. said yesterday he is investigating the latest in a series of allegations concerning county Sheriff James D. Swinson. This allegation is to the effect that a county jail inmate helped build a fireplace at the sherriff's private home.

Earlier Swinson's chief jailer testified that two different inmates had helped build a small tool shed at Swinson's new home in Great Falls. The chief jailer pleaded guilty to charges of using a prisoner to do outside work and was fined $25.

Swinson, as he did yesterday, has consistently denied any knowledge of the use of prisoners at his home. Swinson said he did not know if any inmate had built his fireplace.

Several former and current deputies at the jail - all of whom said they feared reprisals if their names were published - called The Washington Post yesterday to say that "things weren't right" at the jail. They said that Swinson, or employes acting on his behalf, used their positions of authority to obtain favors from both subordinates and inmates.

The allegations ranged from the use of prisoners to shine the sheriff's shoes to the use of deputies to campaign for his re-election.

"It is not unusual for the sheriff to bring in the shoes of his family to be shined" by the inmates, one former deputy said. Swinsom laughed at this allegation and said he had inmates shine only his own personal shoes and he paid them between 50 cents and $1 because they needed the money.

Horan said many of the accusations made against the sheriff's department are not violations of any law. These include charges that deputies were urged, if not required, to campaign for Swinson, a Republican, and post campaign literature for him.

Several former deputies said "the word gets around" that they must contribute about $100 apiece to Swinson's campaigns. "When we got off work we were told to pound stakes into the ground at polling places for Swinson and for another Republican," said one former deputy about the 1975 campaign. "This happened more than one time."

Swinson said "I presume some of them did help me. I would presume and hope they would help me," but he said no one was forced or coerced to campaign for him or make contributions.

Former deputies also said food meant for inmates is often turned into treats, such as pizzas, for the sheriff and his staff.

"Inmates would "bring trays of snacks for the sheriff," a former deputy said. "They'd bake pizzas prepared from food for the inmates. I helped myself to those pizzas. It was damn good. I wasn't sure if it was illegal or not."

Swinson laughed at the allegation and said, "I'm not a big pizza man myself," and that if the reporter wanted to know if members of his staff had pizzas she would have to ask them.

The Post printed allegations yesterday that Swinson allowed jail inmates conjugal visits when they should have been in jail and made prisoners do outside work for sheriff's department employes and others, including at least one lawyer.

Former jail employes also told The Post that inmates have been used to prepared food for the sheriff's annual employes' picnic and that food bought for the inmates has been taken from the jail for employes' personal consumption.

Horan said he is investigating the allegations and hopes to complete his report on alleged abuses in the department by the beginning of next month.