Imports of man-made fiber sweaters from Hong Kong, South Korea and Taiwan are being dumped in the U.S. market, the Commerce Department said in a preliminary ruling.

Last year, imports of the sweaters from those sources totaled $1.17 billion, making the case the biggest dumping investigation in years.The dumping margins found by Commerce varied substantially from one supplier to another. Such margins usually refer to the percentage difference between a foreign firm's prices in its home market or "third markets" and what it charges in the United States.

Commerce said South Korean margins were relatively low, ranging from 0.56 percent to 1.17 percent of the sweaters' value.

Most major suppliers from Hong Kong also were found to be dumping by only small amounts. The margins for Crystal Knitters Ltd. and Laws Fashions Knitters Ltd. were so small as to exclude these two firms from the dumping probe, Commerce said.

Most Taiwanese firms, however, were found by Commerce to be selling at much higher dumping margins. Three - Bonanza, Goodman, Knitwear and Nicewear - have been dumping sweaters at a 36.89 percent margin, Commerce said. It found a 33.91 percent margin for New Northern.

U.S. Customs now will require cash deposits or bonds equal to the weighted

average dumping margins cited by Commerce Monday.

Commerce said it expects to make a final ruling in the case by July 5. If it confirms dumping, the case will go to the International Trade Commission for a finding on whether the imports are causing or threatening injury to U.S. sweater makers.

An injury finding would trigger anti-dumping duties.

Textile importers said they would continue to fight the decision, which came as no surprise to them.

"The U.S. dumping law is highly technical and skewed in favor of the domestic industry," said Laura Jones, executive director of the U.S. Association of Importers of Textiles and Apparel.

Seth Bodner, executive director of the National Knitwear and Sportswear Association, which filed the petition that led to the investigation, said, ''Although we believe that the preliminary dumping margins cited today are far too low, we are confident that the Commerce Department's recent investigation based on costs of production will result in the conclusion that sweater manufacturers in these countries have been selling below cost."