Ask an Indonesian the secret of his youthful looks, clear complexion and calm temperament and the chances are you'll be told: "Jamu!"

The traditional herbal medicines of Indonesia, called jamu, are the devoutly trusted remedies and tonics for all classes of Indonesians. Despite the spread of modern medicine and the opening of some 44 modern pharmaceutical and cosmetic ventures in the last 10 years, the makers of jamu are experiencing a boom.

Solo, the central Javanese town of crumbling palaces and slow-pedaling becaks (pedicabs), is the center of the expanding jamu industry, with enterprises ranging from the gleaming large factory of the Air Mancur Jamu Co. to small shops like Akar Sari in Jalan Coyudan.

Akar Sari seems to be the busiest place in the neighborhood, with delicious smells wafting out from essences such as sandalwood, cinnamon, and pepper. The walls are lined with jars and boxes holding about 150 extracts from fragrant woods, barks, seeds, roots, stems, leaves and berries.

SINDHU WIJANTO, a son of the family that has run Akar Sari for three generations, brings me a glass of beras kencur - a spicy tonic - and says they have jamu for just about everything from back pains and dysentery to masuk angin - a mysterious Indonesian illness that seems to correspond to a chill.

There is jamu for depression and for kidney stones, jamu for high blood pressure and drooping libidos.

Sexual problems are in fact a major field for the jamu industry. Millions of Javanese men regularly take a special jamu, advertised "for those who have the wish but not the capability." Dosage ranges from a glass a week for the 20-year-old to a glass every day for the elderly.

The effective ingredient for most of these tonics is pasak bumi , a root found along the Barito River in central Kalimantan. The Dayak people, who discovered the root, add hot spices to is as well.

Another Barito River herb is used to increase the interest of Javanese ladies, who already will have taken a jamu of their own to put on or take off weight, make their bodies curvier, banish body odor and freshen their complexions.

For village women not yet reached by national family planning, one jamu induces abortion, although it is now sold and advertised as being only for regulating menstruation.

According to Wijanto, business has never been better and customers include many educated young people.

CENTRAL JAVA'S provincial capital, Semarang, is also seeing an investment boom in the product. One company just finished a new factory fitted with automatic production lines and a modern laboratory. On a recent visit, I was presented with samples of the latest breakthrough: male strength jamu in modern dissolving capsules.

As with most businesses in Indonesia, few details of company finances are ever disclosed. However, the marketing manager of the Nyonya Meneer Co. in Semarang, Charles Ong, said sales were around $500,000 a year, and that his company was thought to rank third in volume. Ong, who studied business administration in Miami and Melbourne, said sales have increased by about 10 per cent a year.

At the other end of the scale are the thousands of individual jamu sellers who make up their own and peddle it by foot around the worksites of large towns.

The young women of Solo are a familiar sight dressed in traditional sarongs and carrying a basketful of jamu bottles (mostly Johny Walker empties because they are square and pack better) on their back in a batik sling.

LATE LAST year the Indonesian Government introduced the first laws to specifically regulate the jamu industry and appointed and engaging Javanese official, Bambang Sutrisno, to head a new directorate of traditional medicines in the health department.

The new rules require registration of all jamu factories and full descriptions of ingredients on labels. Standards are set for factories, and mixing of modern drugs with traditional recipes is forbidden. All imported traditional medicines are banned, although some raw materials may be brought in.

Sutrisno says the regulations were needbecause some makers had been tampering wirh jamus proven by generations of use. "They were mixing them with modern drugs like novalgin or aspiring," he said, "We hope to eliminate this fake jamu."

Sutrisno said he believed jamu had a continuing role in the treatment of "physiological disturbances," such as infections ordinarily requiring modern antibiotics.

"First you must know the philosophy of jamu," He saie. "The aim is not just for a certain kind of illness but for the whole body.

For example, Sutrisno said, "Take the jamu for kidney stones, but also the diuretic that washes them away. Then there is the jamu that increases the properties of the kidney so that new stones will be prevented from forming."

He said Indonesian doctors turned to jamu during World War II when modenr drugs were scarce. One variety proved effective against hepatitis, possibly becaukse of the vitamins it supplied to the liver.

Sutrisno was clearly waiting for the question about the jamu pasak bumi: was it really an aphrodisiac." "Put yourself in the place of the Dayaks," he said. "If you discovered pasak bumi would you export it." I think we outsiders are cheated and given the wrong kind of herb." What then about the millions of devoted and satisfied customers." "I think it's all a matter of faith, to do with the power of suggestions," he said.