On Dec. 17, 1983, the Greyhound Corp. shut down the 13 Armour Food Co. meatpacking plants it owned and eliminated the jobs of more than 1,500 unionized workers, who were earning approximately $10 an hour.

Two days later, an Omaha-based food conglomerate, ConAgra Inc., purchased Armour and reopened 12 plants with a nonunion work force of about 900 employes, whose pay started at $6 an hour. ConAgra offered jobs to only a few hundred of the former unionized employes.

The actions, which prompted protests from organized labor, have triggered potentially the largest hiring discrimination case in the 50-year history of the National Labor Relations Board. The NLRB has accused ConAgra of illegally refusing to hire union members at the new, nonunion plants, board officials said yesterday.

After more than 18 months of investigation, NLRB General Counsel Rosemary Collyer has decided to issue a complaint against ConAgra that could result in a rehiring order and a back-pay award of more than $30 million if the meatpacker is found guilty of discriminating against 600 to 800 former employes, according to NLRB officials.

The Armour case has been closely followed by labor and management because it is a major example of a unionized enterprise becoming nonunionized under circumstances that prompted a legal challenge. Unions have accused the NLRB of unfairly interpreting the National Labor Relations Act in favor of employers, a charge that board members have disputed.

The complaint must first be heard by an administrative law judge, whose decision likely will be reviewed by the five-member board, probably next year.

The United Food and Commercial Workers International Union (UFCW), which initiated the case, charged that ConAgra intentionally sought a majority nonunion work force because if it had reopened its plants with a majority of union workers it would have been legally obligated under the NLRB's "successorship doctrine" to honor the UFCW-Greyhound contract, union lawyer David Rosenfeld said.

"These allegations, in our view, are totally without merit, and we will continue to contest it," said Walter Casey, vice president of ConAgra, which has 30,000 employes and annual sales of more than $5 billion. "Our policy was very, very clear that we wanted to employ a work force without discriminating for or against former employes."

ConAgra was willing to recognize the union, he said, but the members twice vetoed proposed pay cuts that ConAgra wanted before assuming the UFCW contract.

NLRB regional counsel Walter Kintz said that Collyer's decision, reported yesterday in the Los Angeles Times, was told to the parties last week; and the NLRB offered ConAgra a chance to settle the case by negotiating with the UFCW before the complaint is issued.

But Casey said ConAgra believes "settlement is not an issue when the case has no merit." UFCW spokesman Al Zack said that the union plans to pursue its boycott of Armour products and other strategies to pressure the company to recognize the union.

Because ConAgra bought Armour's assets -- its plants and brand name -- rather than its stock, the firm was not obligated to follow its union contract, Kintz said. But the company was obligated "not to discriminate" in hiring, he added.