Sen. Edward M. Kennedy (D-Mass.) and the American Medical Association, arch foes on many health issues, agreed yesterday that the United States has no national strategy to prevent disease and ought to develop one.

Kennedy and the AMA jointly opened a three-day conference on "positive health strategies," at which AMA and other health leaders, AFL-CIO President George Meany and Health, Education and Welfare Secretary Joseph A. Califano Jr. will exchange what are expected to be some clashing views.

Kennedy heads a Senate health subcommittee that has often been harsh on organized medicine and he is the leading congressional advocate of national health insurance. Such a program is anathema to the AMA.

Last January, however, the AMA asked Kennedy to speak to its annual leadership conference. Dr. Tom E. Nesbitt, AMA president, said Kennedy urged cooperation between the senator's staff and the AMA.

That led to the conference, which continues at the Shoreham-Americana Hotel today and tomorrow, with the cosponsorship of HEW, the AFL-CIO and 14 other groups.

The main aim, Nesbitt said, will be to look at areas where we should concentrate our resources, public and private," to prevent smoking, obesity, excess alcohol consumption, accidents and stress, all of which lead to deaths and disease.

In a prepared statement to the conference on its opening day, Kennedy called for vigorous enforcement of laws to guard the workplace and environment against poisons, for basic biomedical research on disease prevention, for studies of ways and apply research knowledge and - in a hint of his health financing philosophy - for making preventive services like prenatal care "widely and easily available," with no one denied them "because they can't afford to pay."

Kennedy urged the Senate to pass a disease prevention bill to convert a current $150-million-a-year federal program to aid state and local health departments into one that would total $350 million in fiscal 1980 and reach $540 million in fiscal 1982. The AMA said it supports the bill, "with minor changes."