LOS ANGELES, JULY 26 -- Defense attorneys said today they have proof the U.S. Coast Guard withheld for a year evidence that could have cleared former captain Joseph Hazelwood of charges of operating a vessel while intoxicated, leading to the nation's worst oil tanker spill.

Hazelwood attorney Thomas M. Russo said a certificate dated April 25, 1989, and signed by the director of a California testing laboratory shows the Coast Guard was sent a copy of a chain of evidence form that indicates mishandling of Hazelwood's blood samples.

The Coast Guard did not pass the form on to Hazelwood's attorneys until two months ago, long after he had been publicly lampooned as the drunken captain of the Exxon Valdez and been tried, unsuccessfully, in Anchorage on intoxication charges the form might have rendered invalid. "We're dealing with some type of coverup here," Russo said.

Coast Guard spokesman Cmdr. John McElwain said he had no immediate comment on the charge. A Coast Guard attorney Wednesday denied any coverup, saying he had not seen a copy of the disputed form until less than two months ago.

The ChemWest Analytical Laboratories form, dated four days after the Exxon Valdez struck a well-marked Alaskan reef and spilled nearly 11 million gallons of crude oil on March 24, 1989, shows the laboratory reported receiving Hazelwood's samples in three red-stoppered 10-milliliter tubes, rather than one red-stoppered tube and two grey-stoppered tubes, none of them 10-milliliters, as recorded by the Coast Guard health technician who took the samples. The discrepancy suggests a break in the chain of evidence that could invalidate the laboratory's report that Hazelwood had a .061 percent blood alcohol level 10 1/2 hours after the mishap.

Drucella Andersen, spokeswoman for the National Transportation Safety Board, said her agency received a copy of the disputed form in April 1989 but was not aware of the discrepancy and its significance for the trial. CompuChem resisted providing information for Hazelwood's January-March trial, citing privacy rules and the need for federal approval. The laboratory's director Michael A. Peat testified under court order but said nothing about the discrepancies.

Kees Verkerk, chief executive officer of North Carolina-based CompuChem Corp. which owns the ChemWest laboratory in Sacramento, said today an employee was apparently confused by red adhesive tape on top of the tubes and corrected her error a month later when rechecking the samples.

Russo said today Verkerk's account failed to explain why other samples in the same shipment were correctly recorded and why the ChemWest employee also reported tubes of a different size than those used by the Coast Guard technician.

Verkerk said CompuChem was compiling a chronology in the matter. The company performs drug and alcohol tests for 50 percent of the Fortune 100 companies and is certified by several federal agencies.