Before a crowd of incredulous students, Alexander Calder, the American sculptor famous for his mobiles, supervised the first stages in the assembly of a 40-foot-high "stabile" (fixed mobile) at M.I.T. yesterday.

The 33-ton steel sculpture, when completed in three days, will look like a cluster of black sails, but it represents nothing in particular. "I just built it," Calder said.

Calder seemed to be enjoying the flurry he was creating as he shook hands and consulted with the workmen. On top of his bushy white hair, he sported a Turner Construction Company hardhat. Under his overcoat was a magenia silk ascot.

When someone asked the massively-built old artist what he had designed before creating M.I.T.'s big sculpture he laconically replied, "small ones," and burst into gales of laughter.

Construction workers are using a 60-foot crane to lower the 35 plates of the spikey structure into place. Three thousand pounds of nuts and bolts will hold the pieces together.

The French technician who helped Calder cast the plates in France estimated the cost of the sculpture at 15 to 20 million old francs. "In dollars zat ees.." he began, and then gave up computing. Someone else said that the huge object d'art probably cost around $ 40,000.

M.L.T. students were not impressed.

One architecture major suggested painting the sculpture purple. An entrepreneur-type offered to trade the sculpture for the statue of John Harvard in front of University Hall.