The Supreme Court ruled unanimously yesterday tht U.S. Steel Corp.'s method of loaning money to housing developers buying the company's prefabricated house does not violate the antitrust laws.

The ruling apparently ends at 13-year lawsuit brought by a Louisville housing developer who claimed U.S. Steel illegally tied favorable credit terms to his purchase of company homes at above-market prices.

The case had been to the Supreme Court three times, and the justices ruled 5 to 4 in 1968 that the developer was entitled to a trial to attempt to prove U.S. Steel violated the Sherman Act by tying in credit to the purchase of homes.

At that time, businessmen expressed fear that the justices eventually might hold that even department store credit-card arrangements are illegal tie-ins. Yesterday's decision did not reach those issues directly but seemed to imply that credit terms alone may be insufficient to find antitrust violations.

Under the Sherman Act, findings of illegal tie-ins essentially require that commodity on purchase of a realily available item. Essentially, the seller then is using his monopoly position in one commodity to force sales of the common commodity as well.

There was no dispute in the current case that prefabricated homes were readily available from other companies, usually at lower prices. But the developer said U.S. Steel used its immense financial power to offer loans at below prevailing interest conditioned on buying its houses. justice John Paul Stevens wrote that there was no uniqueness to the credit terms, however.

In another case, the court handed labor unions a major defeat in ruling that lawful job preservation rules in a union contract are no defense to an unfair labor practice charge when employees refuse to handle prefabricated products required by general contractors.

The majority said a union is guilty of an illegal secondary boycott when it orders employees not to handle prefabricated goods unless the subcontracting employer is able to order a change.

The ruling drastically cuts union power to strike in the construction business because it bars work stop-pages to enforce work preservation clauses when the architect or general contractor is bound to use prefabricated work.

The court also agreed to decide if the owner of a building leased to another company may take federal income tax deductions for depreciation of the building and interest on loans for constructing it. The justices will review a U.S. circuit court decision disallowing such deductions.