John Sneed's farm is either a weed Waikiki or a hay fever sufferer's Hades.

Ragweed and lamb's quarters flourish on this hilltop 90 miles east of Kansas City, spreading their leaves luxuriously in the sun and sinking roots deep in the sandy loam. Pigweed, horsetail and golden rod, unmolested by chemicals and cultivators, mature to robust adulthood, producing seeds that are spread by the wind.

Indeed, Mr. Sneed encourages all his weeds to breed. Long ago Mr. Sneed discovered that weed pollen is worth a considerable amount to a handful of pharmaceutical manufacturers, who turn it into medications to diagnose and treat allergies. Some weed pollen sells for $6 a gram. In 1986 Mr. Sneed and two sons turned this runny-nose harvest into nearly $150,000 in cash. Along with the soybeans, corn and livestock they produce, the Sneeds' 800-acre farm earns enough to support three families.

''The dictionary defines a weed as an unwanted plant,'' said Mr. Sneed as he gingerly stepped across rows of milk weed and English plantain he sowed several weeks earlier. ''As far as I'm concerned we don't have any weeds because I want them all.''

Across the country, family farmers are turning away from traditional crops and livestock and finding new ways to earn money. Mr. Sneed, though, is a rare breed who has discovered the delicious greed in weeds. ''He's the only commercial weed farmer I know of, and I would have heard if there are any others,'' said George M. Coleman, director of manufacturing for Hollister Stier, an allergy medicine manufacturer in Spokane, Wash., and a division of Milses Incorporated, a pharmaceutical company in Elkhart, Ind.