Velico Chemical Corp., a veteran of numerous battles with environmentalists and federal regulators, was indicted yesterday on charges that it concealed laboratory tests indicating that two of its bestselling pesticides might cause cancer.

A federal grand jury in Chicago, were the company is based, said Velsico willfully withheld from the Environmental Protection Agency warnings from three pathologists about the pesticides heptachlor and chlordane. Six present or former Velsico employees and consultants were also indicted in the case.

Heptachlor and chlordane were banned for most purposes in 1975 when the EPA concluded that they posed an "immediate hazard" of cancer in humans. Until then they had been widely distributed for use both in large-scale lamging and around the house.

Velsico continued to export heptachlor and chlordane after their domestic uses were banned.

Velsico is the only producer of the two pesticides, according to EPA. The firm, a subsidary of Northwest Industries, Inc., has also manufactured such controversial compounds as Tris, a flame-retardant banned last year as a potential caroinogen, phosvel, a nerve-destroying pesticide that has never been approved for sale in the United States, and endrin another pesticide that had been investigated by EPA as tumor producer.

Federal law requires pesticide producers to provide the government all the information the manufacturers gather about possible hazards from their products. Federal officials said yesterday's indictment was the first criminal prosecution for failure to provide such information.

In a statement yesterday, Velsico said the case involved "delay" in reporting certain laboratory findings to EPA. The firm's statement said those findings were "insignificant fragmentary and incomplete." It called the indictment was an "outrage."

Heptachlor and chlordane were among the first generation of modern chemical insect killers developed after World War II. They were used to combat soil-boring insects on major feed crops, such as corn, and were also sold in hardware stores for home use against roaches, beetles, and termites.

The two compounds were considered valuable because of their chemical stability - they did not break down into different compounds. But that virtue became a frightening vice when the federal government began investigating the possibility that the two pesticides might cause cancer.

Because of their stability, the two chemicals were passed intact up the food chain. A Food and Drug Administration sampling found heptachlor and chordane in 73 per cent of dairy products meant for human consumption and in 77 per cent of meat, fish and poultry products.

Rachel Carson warned about the two compounds in her 1962 book, "Silent Spring." Velsico tried, through a letter from its lawyer, to prevent publication of the Carson warnings.

In 1972, EPA began an investigation to decide whether it should open formal proceedings to ban certain uses of heptachlor and chlordane.

Velsico, with EPA's approval, hired an independent laboratory to test the effect on mice of the two compounds, according to the indictment. The manufacturer retained three independent pathologists to review the findings of the laboratory study.

The indictment says that Velsico forwarded to EPA some of the results of those studies but concealed other results that indicated serious hazards from the two compounds.

EPA learned later about the allegedly concealed warnings. Officials at the agency said they referred the case to the Justice Department for prosecution. In yesterday's indictment, the firm and the six individuals were charged with conspiracy, mail fraud, and defrauding the U.S. government.