On top of the 1250 alumni flooding into Cambridge for the post-Victory reunion of the Associated Harvard Clubs, individual class reunions, principally the twenty-fifth of the Class of 1921, are luring additional hundreds back to the Yard. Nearly 300 members of '21 were registering at Eliot House headquarters yesterday and this morning for the first full-scale twenty-fifth convention since 1941.

Other prominent class reunions include those of the Classes of 1901, 1926, 1931, and 1936 and will feature the customary round of cocktails, clambakes, luncheons, hand-shaking, back-pounding, and haven't-seen-you-sinces.

Meanwhile, five classes will hold reunions in conjunction with the Associated Harvard Clubs' doings. These include the sixtieth, fiftieth, fortieth, thirty-fifty, and thirtieth reunion classes: 1886, 1896, 1906, 1911, and 1916.

Activities of the twenty-fifth reunion class will get off to an athletic start today with golf, tennis, softball, croquet, swimming, sun bathing, and launch trips at the Essex County Club, Manchester. A memorial service in Memorial Church, the Yale baseball game, dinner, and a dance are on tomorrow's program, with Commencement Exercises, a noon spread, alumni exercises, and tea completing festivities Thursday.

For '86, a Class Dinner at the Harvard Club of Boston tomorrow evening will highlight proceedings, while a more elaborate agenda of cocktails, luncheons, and dinners is planned for '96.

Spreading themselves beyond the realm of Greater Boston, '01 and '26 reunions will center around the Cliff Hotel at Scituate, while '31 will go as far afield as Poland Spring, Maine, for its fifteenth. Members of '31 left yesterday noon on special cars of the "Flying Yankee"; they will return tomorrow in time for the baseball game.

Wentworth - By - The - Sea, Portsmouth, New Hampshire, will be the scene of '36 reunion festivities; the class will leave for Portsmouth this morning and expects to return for lunch tomorrow.