The Supreme Court left standing yesterday a first-of-its-kind ruling that a taxpayer can invoke a Reconstruction-era civil rights law to sue a state or county tax official who intentionally or recklessly over-assesses property owned by an individual or a business.

The case came from Illinois, where state law required all property -- commercial as well as residential -- to be assessed at 100 percent of market value from 1958 to 1971, and at 50 percent from 1971 to 1974.

In Chicago, Cook County Assessor P.J. Cullerton was generally assessing residential properties at between 20 percent and 35 percent of actual value.

He said repeatedly and publkicly "that his assessments intentionally discriminated in favor of homeowners," said the successful plaintiff in the case, Fulton Market Cold Storage Co.

The defendants argued that the suit was barred by the Federal AntiTax Injunction Act, which prevents federal courts from interfering with state or local taxation if state courts may provide "a plain, speedy and efficient remedy."

The court took other actions:

Private "Mail" Service

Three years ago, Patricia H. Brennan and her husband, J. Paul Brennan, started a service in downtown Rochester, N.Y., in which, for a dime, they'd pick up a business-size envelope before noon and deliver it the same day. The charge: ond dime. By last July, they had 357 customers, eight full-or part-time employes, and a volume of 3,000 envelopes a day.

Yesterday, however, the court let stand a ruling that Congress had given a monopoly on conveyance of "letters and packets" to the Postal Service.