Spurning medical advice that he was risking his life, United Farm Workers' union leader Cesar Chavez continued his fast into its 28th day Saturday, but grape growers say his effort has attracted more publicity than actual support for the union's 4-year-old boycott.

The image of a frail Chavez shuffling into the union meeting hall to attend Mass has spurred unions from all over the country to send telegrams in support of his open-ended fast.Hollywood stars and children of the late Sen. Robert Kennedy came to this rural community 150 miles north of Los Angeles and made dramatic endorsements of the boycott, and the president-elect of Mexico, Carlos Salinas de Gortari, issued a statement on behalf of Chavez, 61.

What remains to be seen is if publicity raised by the longest fast of Chavez's life will inspire consumers to stay away from California table grapes. He conducted a 25-day fast in 1968 and a 24-day fast in 1972.

Two decades ago, Chavez had his first major success with another grape boycott. The highly publicized, five-year boycott ended when Delano-area growers agreed to a contract with the union in July 1970.

But the UFW no longer has any contracts with California table grape growers, and its dues-paying membership has been halved since 1978, to about 30,000.

During the current boycott, one California supermarket chain, Save Mart, has obtained court orders to stop picketing outside its stores. Officials of other California grocery stores and restaurants report little consumer action on behalf of the boycott.

"I'm not aware of any impact directly related to the food service industry," said Stanley Kyker, executive vice president of the 9,000-member California Restaurant Association.

"The boycott, as far as I'm concerned, has been nothing," said Charles Collings, president of Raley's Supermarkets and chairman of the California Grocers Association.

Since beginning his water-only diet July 17, Chavez has lost at least 26 pounds and barely has the stamina to sit through a two-hour nightly community meeting and Mass. He suffers bouts of nausea and dizziness, and he nearly fell Wednesday after standing for a prayer.

But despite those problems and a risk of kidney failure, he refuses his doctor's request to end the fast and enter the hospital. Chavez's mental condition reportedly remains sharp as he spends his time largely in bed, reading works of Martin Luther King Jr., St. Francis of Assisi and Gandhi.

The boycott has two goals: to halt the use of five pesticides and protest the state's Agricultural Labor Relations Board, which oversees farm labor elections. The union says the agency has become a tool of the administration of Gov. George Deukmejian.