An undergraduate and a Law School alumnus subdued a 19-year-old, knife-carrying youth in the Radcliffe Quad Saturday night as he was attacking a Radcliffe girl in the bushes outside Cabot Hall.

George Stanley Robinson, a criminal lawyer from Vienna, Va., pulled a wooden field hockey goal support out of the ground and knocked the youth unconscious. A few seconds before, J. Manchester Robinson '61-3, no relation, had jumped the youth and pulled him away from the girl.

Earlier, the youth slashed the hand of a Lowell House student, Thomas L. Robbins '65, in a scuffle near the Boylston Hall entrance to Harvard Yard. Robbins was treated at the Health Center for a cut palm.

The younger Robinson who edits a literary magazine, said he had been attracted by commotion near the Yard and saw Robbins crossing Massachusetts Ave., his hand bleeding. He said the youth, knife in hand, started toward Garden St., staggering as if drunk. As he crossed the street he lunged at a passing cyclist with the knife.

The slight, blond-moustached Robinson, asked several cab drivers for help, but the drivers shrugged. He then followed the youth up Garden St. Near the Unitarian Church cemetary, the youth began to follow two girls, keeping within a step of them.

Frightened, the girls quickened their pace, but the youth kept up with them.

Robinson met George Robinson, who was in Cambridge for his 35th Law School class reunion, at Shepard St., and they continued the pursuit. By this time the youth had exposed himself.

The girls entered the quad, followed by the youth. Near the bushes he grabbed one of them by the arm, threw her into the bushes and jumped on her. The younger Robinson said he sprinted toward the bushes, pulled the youth away and got him in a full nelson. Seconds later, as the youth screamed "I'll kill you", the lawyer hit him on the back with the heavy goal support and knocked him cold.

As the girls streamed from the dormitories, the undergraduate bound the youth with a belt. Police and the Cambridge Rescue Squad arrived and the youth was taken to Cambridge City Hospital. He suffered no internal injuries or broken bones. Cambridge police said yesterday that the youth would be arraigned today in the Third District Court.