James M. Stone '69, lecturer on Economics, was sworn in yesterday afternoon as Massachusetts commissioner of insurance.

Stone, age 27, is likely to be the youngest commissioner in the new administration of Gov. Michael S. Dukakis. He now teaches Economics 1301. "The Economics of Security Markets," and works as a vice president and economist at Fairfield and Ellis, a Boston insurance firm.

Yesterday's appointment follows several days of speculation about whether the $25,228 position would go to Stone or to State Rep. Laurence R. Buxboum (D-Sharea), who was seen two weeks ago accepting congratulations from his colleagues in the legislature.

Stone, who declined yesterday to discuss his new position until he has had more time on the job, described himself as "proconsumer." "But I would remind everyone," he said, "that one part of helping the consumer means making sure the insurance companies in this state are strong.

Stone replaces John G. Ryan, an appointee of former Gov. France W. Sargent.

After the official announcement, Stone said, he "went to the office, and I brought home some papers to look over tonight before going back in tomorrow."

Stone graduated from Harvard summa cum laude, won the Young Prize for "the best undergraduate thesis of summa quality," and won the Goldsmith Prize for the best research or seminar paper presented to the graduate school of Economics.

He has worked for Fairfield and Ellis since 1971 and since 1973 has divided his time between his executive position there and at Harvard