The Interior Department is "seriously considering" a proposal to return ownership of 50 acres in Wisconsin's Apostle Islands National Lakeshore to the family that sold it to the government two years ago, according to several officials.

While environmentalists denounced the proposal as an erosion of the national park system, Interior spokesmen said it would benefit the popular lake shore by increasing public access to the land.

"It would be an exchange of basically equal values," National Park Service spokesman George Berklacy said.

"The public would get the access, and the owners would get the title" to the land.

Berklacy said the Anderson family, which owned the land, retained control of its use when the property was sold to the park service in 1981. This control could last as long as 70 years, he said, and restricts public use of a harbor and other areas of the island.

Interior has made no decision on the proposal, according to Berklacy, but other officials said it is likely to be approved.

Interior's willingness to consider the proposal has drawn an angry reaction from environmentalists and Wisconsin politicians who supported the bill creating the 42,000-acre national lake shore "to preserve for posterity a rare and priceless resource of natural beauty."

If approved, the plan would create a private holding within a park that Congress directed the Interior Department to purchase, a precedent that environmentalists contend could be extended to other units of the national park system.

"It appears to be a matter in which a special accomodation is being considered for one private interest who has preferential access to Interior Department officials," Wisconsin Democratic Reps. David R. Obey and Robert W. Kastenmeier wrote yesterday in a letter to Interior Secretary James G. Watt opposing the proposed transfer.

Former Wisconsin senator Gaylord Nelson, sponsor of the law creating the lake shore and now chairman of the Wilderness Society, called yesterday for a congressional investigation into what he termed "gross abuse of administrative discretion."

Interior officials denied that the Andersons, owners of the Anderson Window Co. in Minneapolis, had no special access. But they said the family made direct contact with Ric Davidge, a deputy to Assistant Interior Secretary G. Ray Arnett, who oversees national parks. Davidge summoned the lake-shore superintendent here for a meeting on the matter Jan. 10, they said.

The Andersons testified against the legislation creating the park and fought government efforts to purchase their land, according to a family member.

They agreed to sell in 1981, after the park service brought a condemnation case against them, but contacted Davidge in hopes of regaining title to the property, according to the family member who asked not to be identified.