William Ruckelshaus, the new administrator of the Environmental Protection Agency, fingered the plexiglass block just presented to him as a going-away gift from the Urban Institute, the think tank whose board he has chaired for four years.

"I assume," Ruckelshaus said last night as he examined the clear object, "there are no toxic substances in this"--the guests gathered in an airy room of the International Club on K Street guffawed--"and also that whatever monies you used to buy this have been laundered."

Ruckelshaus was greeted in many quarters this spring as a White Knight on a mission to put things right at the EPA, where he served as the agency's first administrator 13 years ago. The Urban Institute, a nonprofit research group specializing in social and economic problems, has surrendered Ruckelshaus graciously.

"I always feel you have to be able to give up the best to the government," William Gorham, president of the institute, said. "And then, get it back."

Ruckelshaus, for his part, is enjoying the new job. "I've only been sued once," he said, smiling. "I got sued five hours after I got there--one of those groups in New York State worried about PCBs in the Hudson River." He turned to Gorham. "What do you think of PCBs, Bill?"

"Never been for them," said Gorham.

The group assembled for the party numbered about 60. They were administration officials, Washington friends, institute staffers and trustees--many from the category of Republicans admired as nonpartisan--including former Housing and Urban Development secretary Carla Hills, former transportation secretary William Coleman and former attorney general Elliot Richardson.

Hills will now chair the board of the institute. "Determined to keep it in the family," quipped presidential counselor Edwin Meese to Ruckelshaus. "That's great."

Though Hills is a Republican following a Republican, she noted, "A lot of groups will slant a product to fit the customer, and I've never known the Urban Institute to ever do that--to any customer."

Richardson, whose ties to Ruckelshaus include the Watergate scandal's Saturday Night Massacre in 1973, said of his friend's return, "I've encouraged him to do this from the beginning."

Less in prominence last night were representatives of environmental groups--generally tough critics of thc administration's environmental policies and practices so far--though several were invited. The National Wildlife Federation's Jay Hair, executive vice president, and Ben Dysart, president, were among those on hand.

"We were the only conservation group who endorsed Ruckelshaus," Hair said, "and we had been strong critics of Reagan and the EPA. We felt the time had arrived when we had to get out of a confrontational mode with EPA and cooperate. Were these problems with Anne Burford or problems that have a deeper source in the White House? Now, we'll find the answer to that question."