E.I. duPont de Nemours & Co. has doubled its investment in genetic research with a $6 million, five-year grant to the Harvard Medical School, for which Du Pont will receive exclusive rights to make use of discoveries achieved through its financial support.

The grant, announced yesterday, reflects Du Pont's accelerating interest in the development of gene-splicing techniques to produce drugs, agricultural chemicals and related products and new strains of seeds and plant life.

Du Pont, other giant rivals like Dow Chemical Co., Monsanto and Standard Oil of California, and the small, specialized firms like Genentech Corp. and Cetus Corp. are already crowding into the new life sciences-genetic engineering field.

Du Pont's grant to Harvard will support reserach by the new Genetics Department at the Medical School, headed by Dr. Philip Leder. Harvard will hold all patents resulting from the research grant and Du Point will receive exclusive rights to their use through licensing agreements, said a Harvard spokesman.

The company is not setting research goals for Leder, but instead, is interested in contributing to basic research in the molecular genetics field, witht he opportunity to draw on the results, said Iwlliam G. Simeral, a member of Du Pont's executive committee responsible for research policy. After an initial, $2 million installment next month, Du Pont will make annual grants of $1 million through 1985.

In addition to the Harvard grant, Du Pont is spending between $1 million and $2 million annually on genetic research, said Simeral. It is still a small part of the company's overall research budget of $570 million this year, but it is growing, he added.

This year, Du Pont purchased New England Nuclear Corp. at a cost of $430 million in Du Pont stock to get access to the Massachusetts' company's expertise in medical electronic and nuclear research. Along with a number of rivals, Du Pont is investing heavily in the search for using genetic technology to mass-produce interferon, a human protein that is being tested as a potential weapon against cancer and viruses.

Add Du Pont's investment in pharmaceuticals, agricultural chemicals and medical diagnosis materials, and the company's life-science activites account for 10 percent of its overall business.

The acquisition of New England Nuclear was a long step for Du Pont beyond the familiar path of chemicals and fibers that it has followed for 178 years. One of New England Nuclear's main products is a family of test-tube proteins called monoclonal antibodies, formed by the fusian of diseasefighting antibodies with tumor cells. Each antibody, taken from human or animal cells, could be mass-produced by the new hybrid monoclonal cell if the techniques can be perfected.

In this way, scientists hope to be able to make commercial quantities of specific antibodies capable of locating and attacking a target caner cell or virus in the body, rather than trying to fight such diseases with chemicals that don't discriminate between healthy and disease cells. Simeral calls the technique "a bullet instead of buckshot."

In the agricultural area, Du Pont is experimenting with the genetic modification of seeds and plants, said Simeral, citing the possibiliy of equipping plants with bacterial that can convert nitrogen in the atmosphere into nitrates in the soil, making them, in effect, self-fertilizing.

Du Pont chairman Edward G. Jefferson has said that the life-science area "could be a major source of new approaches to making drugs and new approaches in the plant and animal kingdoms over the next quarter-century. It has all the foment that characterized the elctronics and computer explosion."

Du Pont is not the only company that will support Leder's research at Harvard, a university spokesman said. Last year, the faculty rejected suggestions that Harvard become a principal partner in commercal ventures to profit from genetic research developments. But the university changed its policy several years ago to encourage its scientists and researchers to obtain patents on their discoveries, licensing the development rights to outsiders, a spokesman said. So far, such patents have yet to produce royalty income for the university, the spokesman added.