WASHINGTON (AP) _ Criminal defendants who claim innocence and blame each other are not necessarily entitled to separate trials, the Supreme Court ruled unanimously Monday.

The court upheld the federal drug-trafficking convictions of three Chicago- area men, each of whom said he should have been tried separately.

Such defendants should be granted separate trials ''only if there is a serious risk that a joint trial would compromise a specific trial right of one of the defendants, or prevent the jury from making a reliable judgment about guilt or innocence,'' Justice Sandra Day O'Connor wrote.

The decision rejected ''a bright-line rule mandating severance whenever co- defendants have conflicting defenses.''

Federal courts favor joint trials for such co-defendants, O'Connor noted.

In a separate opinion, Justice John Paul Stevens agreed that separate trials need not always be granted. But he said trial judges should decide that issue on a case-by-case basis, adding, ''I would refrain from announcing a preference for joint trials.''

Jose Martinez, Salvador Garcia and Alfonso Soto were convicted of possessing cocaine, heroin and marijuana with intent to distribute and of conspiring to possess the drugs.

Gloria Zafiro. a fourth defendant who did not join the appeal, was convicted of conspiring to possess drugs with intent to distribute.

During the joint trial for all four, Soto testified he did not know that a box he and Garcia took to Ms. Zafiro's apartment contained 55 pounds of cocaine. But Garcia's lawyer said the box belonged to Soto and that Garcia knew nothing about the cocaine.

Likewise, Ms. Zafiro testified that Martinez had brought a suitcase to her apartment but that she did not know it contained cocaine, heroin and marijuana. Martinez's lawyer argued that Martinez did not know the suitcase contained drugs, and that Ms. Zafiro was protecting herself by blaming him.

Garcia, Soto and Ms. Zafiro each were sentenced to 12 years and seven months in prison, while Martinez was sentenced to 21 years and 10 months.