Those wildly popular down clothes, which wearers find high on warmth, are too often low on down content. Horsefeathers? No, but waterfowl feathers -- and even chicken and turkey feathers -- as well as fiber and other materials may be taking more than their share of the fill.

Since last year, when the State of California charged 32 firms with false advertising and unfair business practices in marketing their down products, the situation has improved. Among those cited in this early civil action were manufacturers and retailers including some of the best-known names at all price levels.

Late last year, the United States special representative for trade negotiations, Robert S. Strauss, warned of "massive fraud" in labeling. And last week, a spokesman for the Federal Trade Commission in Colorado -- which has been collecting garments at random from all over the country and testing their down content -- called mislabeling a "widespread practice. We are presently looking closely at several companies and we have issued several subpoenas," the spokesman said.

The FTC action has caused a major flap in the $500-million down-clothing industry. Domestic factories turned out 7 million jackets and vests in 1976 alone, and imports accounted for an equal number. And imports, too, are under scrutiny by the government. In a sampling of $13-million worth of down imports tested by U.S. Customs since last September, 50 percent were found to be mislabeled.

Down -- usually thought of as under-feathers or the plumage of young birds -- is actually a series of small feather-like filaments that radiate out from a central nucleus. Unlike two-dimensional feathers, down is three-dimensional. That configuration is what traps the air between the strands and provides insulation against the cold.

According to government regulations, a down jacket must contain at least 80 percent down and plumules (underdeveloped feathers) and down fiber (broken pieces of the down pod), with not more than 10 percent fiber in that 80 percent.

For the most part, down is a by-product of the goose and duck food industry. After the birds are killed, the down is separated from the plumage by blowing air through it. Down gives the product loft or fill power, and the waterfowl feathers have a curl which assists in lofting, according to James Mills of the FTC here.

The American product is becoming increasingly available as Americans consume more duck in place of increasingly expensive steak and lobster.

But the very best down, according to Ellen Stark of the Feather and Down Association, is from the eider duck, and it is not generally available. Eiderdown is gathered by men lowering themselves by rope to the cliffs where these ducks nest in Scandinavia and Iceland.

Mainland China is a major source of down and down-filled products, perhaps because of its high consumption of duck. But experts are reluctant to predict, at this stage, that the opening of trade with China will in any way affect the status of down imports from that country.

Some weeks ago Sears, Roebuck and Co., prompted by word of FTC and Customs inquiries on labeling of down, undertook their own special testing and found that some items did not measure up to federal standards or their own even stiffer requirements. According to a company spokesman, they withdrew from sale, relabeled and then returned to sale a "very large number" of items whose contents did not meet specifications.

A new label currently attached to a down-like parka at Sears identifies the ingredients of the filler as: 55 percent down, 28 percent waterfowl feathers, 11 percent waterfowl fibers, 6 percent damaged feathers.

Down vests and parkas started to pick up in sales in the 1960s with the back-to-nature emphasis on camping and outdoor activities. Because of the light weight, warmth and virtual indestructibility of the better garments -- Eddie Bauer, The North Face and some other quality jackets are guaranteed for life -- the parka business never let up.

Then, with a renewed interest in rugged outerwear, parkas and vests took another leap in 1976. Even designers Ralph Lauren, Scott Barrie and Perry Ellis eventually got on the bandwagon, putting their own designs on a classic parka, as they more recently have on jeans.

The boom also brought out all the promotional garments at cheaper prices. And when supposedly down parkas started to be advertised at prices below the cost of down itself, the quality manufacturers and the industry associations encouraged the government regulators to take a closer look at the market. And what the government found was a serious infraction of standards and considerable mislabeling.

In a test by the fraud division of the district attorney's office in Sacramento, Calif., last April, of 27 brands of imported apparel items examined, only one brand passed, while the remaining 96 percent failed.

"I think we may have saved an industry," Fred Shippee of the American Apparel Manufacturers Association, said last week. "Those expecting to get performance of down will get what they expect," he predicted confidently.

One alternative to down has been synthetic filling, particularly appealing to skiers whose concern is more for sleek fit than insulation -- and to those who are allergic to down.

"If it was easy to tell a good quality garment from a poor one, we wouldn't be so worried about it," says Shippee.

One thing inhibiting consumers from recognizing quality down fill in jackets is the popular heavier outer shell that makes it difficult to test the fill underneath.

"If that's the case, check out the jacket through the lining," says Jack Gilbert, vice president of The North Face Co. He offers these suggestions for choosing a good garment.

Since loft and thickness provide the warmth (and the high price), look for evenly distributed thickness throughout the jacket, including the area over the shoulders.

Feel for quills. There should be a few, but very few, since the quills are on the feathers, not the down.

Quilt lines (stitching) should be between four and six inches apart since quilt lines are unprotected and are in fact cold spots in the garment.

Check for insulation over the pockets.

Check for adjustable ventilation -- snaps on cuffs or double zippers for example -- since jackets are often so warm that some ventilation is welcome.

Some down jackets are now being sold with samples of recommended down cleaning substances attached. Others recommend a mild detergent -- but never bleach.After cleaning, experts recommend tumble drying with a clean sneaker tossed into the machine. It curbs the static electricity.