The Texas Department of Health has begun an unprecedented study to determine if graduates of a high school located downwind from a petrochemical complex here suffer from an unusually high incidence of leukemia.

The Health Department this week confirmed that three male graduates of Port Neches-Groves High School between 1964 and 1974 have died from leukemia. Authorities are investigating whether a fourth graduate may also have died of leukemia, a department spokesman said.

The National Institute of Occupoational Safety and Health has turned up 11 deaths involving leukemia among workers at the two synthetic rubber plants, said NIOSH project driector Dr. Theodore Meinhardt.

Benzene, a chemical which Meinhardt said has been shown in many studies to be a leukemia-causing agent, is a building-block element of styrene. Butadiene is combined with styrene to form the latex that is the essential ingredient in synthetic rebber, he said.

He said the deaths might be attributable to those ehemicals, or to a number of other chemicals to which the workers were exposed, or to other factors.

Meinhardt said that, according tro personnel records from the two plants, none of the Port Neches-Grbves high school leukemia victims had ever worked at either of the factories.

State Health Department spokesman Lon Gee said, "Studies of the cause of cancer are extremely complex and are undertaken only if there is clear evidence of a significant increase in incidence."

Gee said that data available from state death certificates indicate "slightly elevated" leukemia mortality rates in the area. Meinhardt said three of the five counties included in the investigation are among the top 10 percent of U.S. counties in leukemia deaths among white males.

Gee said that no conclusions about the incidence of leukemia among Port Neches-Groves graduates can be drawn from the three confirmed deaths.

However, William Townsley, the Beaumont, Tex., attorney for former employes at the plant who supplied the initial information about the deaths last week, said they suggest an unusually high incidence of leukemia in an age group not normally susceptible to the disease.

He said that rough estimates he prepared from available cancer statistics indicated that there should havbe been an average of only about 1.4 leukemia deaths among males 19 to 34 years old in the entire county between 1964 and 1974.

Dr. Marcus Key, a former NIOSH director and a professor of occupational medicine at the University of Texas School of Public Health in Houston, said he examined Townsley's figures before they were sent to the state health department.

"I did look at the data, and it struck me that four cases of leukemia among recent graduates seemed on the high side," he said. "It may be a fluke, but I think it should be looked at."

A B.F. Goodrich spokeman at company headquarters in Cleveland, Ohio, said that its Port Neches plant is "the subject of a continuing study by NIOSH, which has not reported any carcinogenic problems connected with plant employment."