WASHINGTON— FOR many years, Social Security cards carried an admonition that they were to be used ''for Social Security and tax purposes -- not for identification.''

That assurance rings hollow today. Congress has authorized so many uses of the nine-digit number, and Americans use it for so many unauthorized purposes, that it has just about become a national identifier.

For the Clinton Administration, which is considering assigning all Americans a health-care identification number, the slow, steady march of the Social Security number from secrecy to all-purpose identifier may be a cautionary tale.

When Franklin D. Roosevelt signed the Social Security Act in 1935, it did not mention the use of Social Security numbers, but it did authorize some type of record-keeping scheme. A year later, the Treasury Department ordered the issuance of an account number to each worker covered by Social Security to keep track of earnings, collect payments and pay old-age benefits.

Even then, there were privacy concerns. ''There was a lot of labor strife in the 1930's, a lot of employer opposition to unions, and when Social Security numbers were first issued, there was concern that they not be used for blacklisting union organizers,'' said Robert M. Ball, a former Commissioner of Social Security.

In 1943, an executive order required Federal agencies to use Social Security numbers whenever they set up new record systems to identify individuals. But in the 1940's and 1950's, the Government made relatively little use of the numbers. That began to change in 1961, when the Civil Service Commission adopted Social Security numbers as the official identifiers of Federal employees. A year later the Internal Revenue Service began using the numbers to identify taxpayers.

But use of the numbers really exploded with the computer revolution of the 1960's. Federal, state and local governments, banks, credit bureaus, hospitals and colleges found that the numbers were handy for keeping track of people in their computers.

In 1972, Congress, concerned about welfare fraud and illegal immigration, directed the Social Security Administration to issue numbers to people who receive or apply for Federal benefits, and to legal aliens when they enter the country.

And Congress has repeatedly authorized new uses of the number, requiring people to disclose their number to get welfare, Medicaid, food stamps, Government loans and other Federal benefits.

In 1976, Congress authorized states to use Social Security numbers in administering ''any tax, general public assistance, driver's license or motor vehicle registration law.''

Better enforcement of child-support orders was the goal in 1996. Under that year's welfare law, states were required to record the Social Security number of anyone seeking a marriage or occupational license, as well as anyone named in a divorce decree or child support order.

By 1996, things had come full circle. In setting uniform standards for driver's licenses, Congress decreed that Federal agencies may not accept licenses as proof of identity unless they include a Social Security number.