Philip Morris, trying to stem the slow decline of its best-selling cigarette, plans to introduce a version of Marlboro called Marlboro Medium that has tar and nicotine levels between those of the regular and light versions.

The new cigarette is intended to slow the defection of Marlboro's own smokers. Marlboro continues to sell strongly in a declining industry and its rich profits helped finance the acquisition in the 1980's of General Foods and Kraft, which made the Philip Morris Companies the world's largest consumer-products company.

But smokers of regular Marlboros who wanted a lighter cigarette disliked Marlboro Lights for taste and packaging reasons, according to industry researchers. Many of them defected to other brands manufactured by Philip Morris and other cigarette companies. These smokers are responsible for buying perhaps 1.5 billion packs a year, or two-thirds of the decline in unit sales for Marlboro.

By offering a cigarette with more tar and nicotine than light versions, Philip Morris hopes to keep more of these smokers. The medium cigarette, to be introduced in June, will have 12 milligrams of tar, compared with 16 milligrams in the "full flavor" version and 11 for the Light.

Marlboro Medium would also have eight-tenths of a milligram of nicotine, compared with one milligram for the regular and seven-tenths for the Light.

When Marlboro Lights were introduced in 1972, they were intended not to hurt sales of the main brand, known as Marlboro "Red," which tended to attract young men, the most desirable customers in the industry since they are the heaviest smokers.