Almost a year after the Amalgamated Clothing and Textile Workers Union signed its first settlement with J.P. Stevens & Co. at a cost of $30 million and 17 years of effort, the union has lost its first representation election in a Stevens mill.

The union's setback came at the Stevens denim fabric manufacturing plant in Rock Hill, S.C., where workers last Friday voted 433 to 299 to deny ACTWU union representation rights.

The vote was the first major test of ACTWU's strength within the 76-plant Stevens empire since Oct. 19, 1980, when Stevens signed its first settlement covering 10 of the company's plants in North and South Carolina and Alabama. In return the union promised it would drop a national boycott of Stevens products.

"The door is open, and now we're in," ACTWU President Murray H. Finley said last year after signing the 2 1/2-year contract. "You just can't measure that kind of thing in dollars," he said.

But textile industry officials are saying Rock Hill workers did just that last week when they rejected the union by a 4-to-3 margin.

"Many people at the plant requested copies of the 1980 contract" before the Rock Hill vote was held, one industry source said in a statement confirmed by several others. "They studied the contract and concluded that there was nothing in that contract that they didn't already have," the industry official, who requested anonymity, said.

Both union and non-union Stevens employes earn an average hourly wage of about $6. Both groups receive identical benefits, according to company officials.

Union officials acknowledged yesterday that copies of the agreement were given to the Rock Hill workers--by management, they said--before the vote. But they blame unspecified "election irregularities" for their defeat and say they will go before the National Labor Relations board this week to file charges against the company.

"We're going to claim that the conduct of the company was such that it interfered with the election," said Jacob Sheinkman, ACTWU secretary-treasurer. Sheinkman downplayed the election loss, saying, "It's a lot of baloney" to call the defeat a major blow to the union's organizing efforts.

"This was our first vote at that plant. We've lost many first-vote campaigns. In this one, we got about 40 percent of the vote. . . . With a showing like that, we're going to be back."

It may be another year before ACTWU gets that chance. Under NLRB regulations, a union has to wait at least 12 months before it may hold an election in a plant where it has been rejected.

Stevens spokesman James Franklin denied that the company--judged by the NLRB to be guilty of unfair labor practices in its dealings with ACTWU on 22 past occasions--was guilty of any wrongdoing in its conduct of the Rock Hill vote. "The company did absolutely nothing wrong," Franklin said.

He said the vote went against the union largely because of economics.

"Many of the workers have very real fears about the future of the industry," Franklin said. For example, Stevens employes are aware that the denim market "is in bad shape" and is running into tough competition from foreign manufacturers.

Stevens normally employs 986 people at its Rock Hill facility, but more than 200 of those people have been laid off because of a downturn in the market. Also, the company announced this week that it was closing one of its plants, in Great Falls, S.C., largely because of market conditions.

The company is abandoning some textile markets--it declines to say which ones--and is investing heavily in labor-cutting automation to increase production and cut costs.

"All of these changes are very important if we are going to remain viable" in the textile industry. But, naturally, they have produced some disgruntlement" in the work force, Franklin said. However, despite the uneasiness, the Rock Hill vote "indicates that most of our employes don't have much confidence that the union can solve their problems and that they're willing to try something else. We're very pleased with the outcome," Franklin said.