UNITED NATIONS, SEPT. 23 -- At road checkpoints all across Lebanon this week, armed militiamen handed out pamphlets proclaiming a revolutionary manifesto -- inoculate your babies -- which, said James Grant of the U.N. Children's Fund, "is better than bullets."

The U.N. agency announced today that the immunization campaign was operating without incident all across the faction-ridden nation, and that in the first two days of the four-day operation, half of the 270,000 children targeted had been immunized against measles, while 75 percent received vaccines against diphtheria, whooping cough, tetanus and polio.

These diseases account for 1,000 to 2,000 deaths and several thousand cases of blindness or crippling in Lebanon each year among the country's 350,000 children under the age of 5. Because the national immunization level has declined drastically over more than a decade of civil war, 270,000 of these children were unprotected until the campaign began.

Grant, UNICEF's executive director, said that the operation would be extended to a fourth day Thursday because of the "quiet and tranquil atmosphere" that has prevailed during the three scheduled days, in hopes that more children will be reached.

The concept of a "truce for the children" was tried with success in El Salvador in 1985. A similar drive in Lebanon was proposed in April, and embraced after painstaking negotiations with all factions from Israeli and Syrian authorities to the Iranian-backed Hezbollah, Grant said.

"The first signal that we could make it work came last Friday," he noted, "when the medical supplies and vaccines were transported to all 29 districts of Lebanon without incident in our own trucks and militia jeeps and armored vehicles, across all checkpoints."