The nation's biggest retailers reported substantial sales increases in March, but cautioned that an early Easter made interpretation of the figures difficult.

J.C. Penney Co., Inc. reported the biggest gain - March sales up 21.5 percent from the previous year compared with increases of 15.3 percent for F.W. Woolworth, 15.9 percent for K mart Corp, and 11.8 percent for Sears, Roebuck & Co.

The increases during March generally ran half a percentage point better than those in February, although Sears' sales growth rate fell from 14.9 percent to just under 12 percent.

Merchants said, however, that it was difficult to tell whether the sales figures meant consumer were spending more, because Easter was two weeks earlier this year.

Much of the Easter bonnet buying last year was done in April, so March and April sales figures will have to be examined together to get a true feeling of the consumer pules. said David Babcock, chairman of the board of the May Department Stores.

May operations, which include the Hecht Co. in Washington and Baltimore, recorded a 7.6 percent increase to $133 million. on a store-for-store basis - discounting the growth from opening more units - Mays sales were up 3.9 percent.

Sears, the nation's biggest retailer, said its sales for the five weeks through April 1 were $1.78 billion, up 11.8 percent.

Although Sears normally gets two-thirds of its sales from so-called hard good (appliances, hardware and the like), the strongest gains this year were in soft goods, reflecting the Easter business. Sears' soft goods sales increased 15 percent over the previous year, while hard goods sales rose only 10.5 percent.

K mart's 15.9 percent gain pushed its sales to $914 million. The number two retailer said the benefits of the early Easter were offset somewhat by the effect of the coal strike.

Penney Chairman Donald Seibert said the Strong consumer demand his chain felt in February continued into March. boosting sales from $712 million to $865 million. Spring fashions paced the sales, but demand was described as brisk in home furnishings, appliances, home improvements, and automotive and sporting goods.

At Montgomery Ward, sales were up 5.8 percent to $423 millon, with the strongest increases coming in the Southwest, Southeast and West Coast, where the impact of the coal strike was smallest. Ward also listed home furniture, auto supplies and appliances as strong sellers.

F.W. Woolworth, which has reported a 5.8 percent sales increase in February, turned in a 15 percent improvement in March, boosting sales to $505 million.

Carter Hawley Hale, the parent of Neiman Marcus, showed a 5 percent gain, to $132 millon for the months, much higher than anticipated, a spokesman said.

Zayer Corp., the big discount chain, reported a 12.8 percent increase $110 million.