A U.S. District Court judge has dismissed the Justice Department from a major portion of a Mississippi prisoners' rights case, accusing the department of taking "inconsistent positions" in the case and of not sharing the interests of the prisoners it was supposed to represent.

"It will serve no useful purpose to summarize the varying inconsistent positions taken by the United States . . . ," said Judge William C. Keady in a ruling Jan. 6 in Mississippi. "The Department of Justice has clearly indicated that the litigation interest of the United States is not in accord with current orders of this court . . . ."

The case was filed nearly 12 years ago, charging overcrowding and inhumane conditions in Mississippi's state prison system. It was brought by private lawyers for the prisoners and Justice later joined in the suit.

After the court ordered the state to correct the situation in 1977, corrections officials began shifting large numbers of state inmates into local jails.

In April, 1981, Justice asked the court to allow federal agents and experts on prison conditions to inspect the local jails in which state prisoners were held.

But since then, Justice has changed its position. The department filed a brief last week questioning the authority of the federal court to order inspections of local jails--since they are not parties to the suit--and saying it would prefer to bring separate lawsuits against local jails providing inhumane treatment.

William Bradford Reynolds, head of the Civil Rights Division, said yesterday, "My sense is . . . that the papers we filed indicated to the court that insofar as the housing of state prisoners is concerned . . . it was really a matter for the plaintiffs to work out with the state. The federal government didn't have an interest in that . . . ."

Justice will continue to participate in aspects of the case dealing with conditions at the state penitentiary at Parchman.

The justice department's withdrawal leaves Ronald Reid Welch of the Mississippi Prisoners' Defense Committee as the only lawyer representing the interests of the prisoners. The committee is a one-lawyer outfit that is rarely sure where to find the next month's rent.

While the controversy over inspection of local jails was going on, 29 prisoners--21 of them state prisoners--died in a fire last November in a county jail in Biloxi. At Welch's urging, Justice had already begun an investigation into conditions in Biloxi, and it was accelerated after the fire.

In last week's ruling, Keady said Justice began filing inconsistent motions in the case on May 20, 1981. That was after Rep. Trent Lott (R-Miss) wrote to the department to argue against federal inspections of the county jails.

On May 19, Deputy Attorney General Edward C. Schmults wrote to Lott that the department would be "satisfied" if Mississippi would voluntarily provide information on conditions and state prisoners in local jails.

The next day, state officials filed the letter with Keady, and he rejected the motion to send federal inspectors into the jails.

On Oct. 21, 1981, Lott wrote another letter to the department, complaining that one of the Justice Department lawyers handling the case, Mary McClymont, had urged the judge to require improved standards in the county jails, and demanding to know if she was obeying department orders.

"If she is, I want to know why administration policy has changed. If she is not, I want to know why she is being permitted to pursue her own policies at taxpayer expense. These are not rhetorical inquiries. I want to know, with reference to chapter and verse of the civil service statutes, why she has not been fired," Lott said.

McClymont has submitted her resignation. She declined to comment yesterday.