Nestle, the world's largest manufacturer of infant formula, has adopted "comprehensive" changes in its "third world" marketing policies which appear to bring it in line with World Health Organization guidelines, former senator Edmund S. Muskie (D-Maine) said yesterday.

But leaders of a five-year international boycott against Nestle products said yesterday that while the changes represented a "few more steps forward," they were still not sufficient to meet the code and vowed to continue their fight.

Muskie, who chairs an outside commission set up by Nestle to monitor the Swiss company's compliance with the WHO infant formula marketing code, said that the company has "responded fully" to recommendations from his commission to improve its sale instructions to Nestle representatives around the world.

The former Carter administration secretary of state said that he would now turn his attention to watching Nestle's "performance in the field" in modifying aggressive marketing tactics that critics say encouraged mothers in developing countries to switch from breast to bottle feeding.

"We will judge Nestle by what it does in the field," said Leah Margulies, who chairs the Infant Formula Action Coalition (INFACT) board, a grass-roots group that coordinates the Nestle boycott supported by more than 100 national groups.

INFACT board member Edward Baer added that he believes yesterday's announcement was a "declaration of intent" in response to the boycott. "We want to see further changes and activities in the field fulfilled. Then we will call off the boycott and turn to other American companies."

Nestle is the only major infant formula company that has outlined its intentions to comply with the voluntary WHO "International Code of Marketing of Breast Milk Substitutes," which was passed by a 118-to-1 vote by the United Nations in 1981. The United States cast the dissenting vote.

After announcing its intention to comply with the code, Nestle sent instructions to its staff interpreting the code earlier this year, but its interpretation came under criticism from the boycott groups as well as the United Nations Children's Fund.

In May, Muskie was named to head a seven-member "Audit Commission" funded by Nestle to police its performance. The commission issued its first quarterly report at a press conference yesterday.

Muskie said that changes Nestle adopted will discourage independent retail marketers from promoting infant formula at any point-of-sale, apply the code for use by children of any age rather than just babies under 5 months of age as critics had charged, provide free or reduced-cost supplies of infant formula only to infants who cannot breast-feed, stop routine provision of infant formula samples to health workers for their own children and clarify that there should be no advertising or other forms of promotion of infant formula to mothers, pregnant women or the general public.

Boycott supporter Baer argued, however, that the company guidelines "do not meet our concerns" about samples, promotion, and advertising. At a separate press conference the INFACT group said that it had recently received claims of 30 violations by Nestle of the WHO code from eight nations.

Muskie, visibly angry about the group's charges, said that his commission had not yet received a single specific complaint and urged the critics to present their cases in detail to his commission.