KUALA LUMPUR, Malaysia (Reuters) -- A government chemist said on Thursday he could not prove that the semen of Malaysia's former deputy prime minister had not been planted on a mattress seized by police to incriminate him of sex crimes.

Lim Kong Boon, testifying in the corruption and sex trial of sacked deputy premier and finance minister Anwar Ibrahim, also told the High Court he had destroyed some key evidence from a lab test on the mattress, which proved that the semen was Anwar's.

Prosecutors said the mattress was seized by police from an apartment allegedly used by Anwar for his trysts in the capital.

They said DNA tests proved the mattress was stained in 13 spots with sexual fluids of the former Cabinet member, two women and two other men.

Defense lawyers have argued that the mattress could have come from elsewhere and that Anwar, beaten unconscious in custody after his arrest, could have had semen drawn from him and smeared on it.

Lim, a forensic scientist in the Malaysian chemistry department, said under cross-examination by Anwar's counsel Raja Aziz Raja Addruse that he was uncertain when the semen stains were made.

"Because it is not possible to determine the age of the stains, it will not be scientifically possible to determine whether the stains came on to the mattress after it had been seized by the police?" Raja Aziz asked Lim.

"Yes, I agree I am unable to determine that scientifically," the chemist said.

Raja Aziz, helped by British and Australian DNA experts brought in by the defense, then tried to discredit Lim's evidence that blood from Anwar matched the characteristic in the semen on the mattress.

But the chemist swore by the accuracy of his analysis, saying Anwar's blood was "very unique" and that the probability of another match between such blood and semen "was one in 59 billion."

Slides were destroyed

Lim said he used microscopic slides to test residues from the mattress to establish the presence of semen.

"Are these slides available today?" Raja Aziz asked.

"No, my lord, those slides are all destroyed," Lim said, drawing gasps from the gallery.

"It's not our procedure in the chemistry department to keep them," he added quickly.

A visibly surprised Raja Aziz then asked the chemist if he had photographed the slides before disposing of them and Lim said that also was not procedure.

"Were you not aware that your findings could have been challenged at any time, and it would have been relevant to keep those slides?" the lawyer asked.

But Lim was defiant. "This is the first time that the defense has asked for the slides used for the spermatozoa isolation process in my 20 years as a forensic chemist."

Anwar, sacked from government in early September, faces five charges each of corruption and sodomy in a trial that has also shaped into Malaysia's biggest political crisis.

Prosecutors allege Anwar used the apartment in the capital to commit adultery with several women and sodomize his adopted brother and ex-driver. Sodomy is a crime in Malaysia.

They also charge that Anwar broke the corruption statute when he ordered police to coerce his ex-driver and a woman to withdraw allegations of sexual misconduct, which they leveled against him in 1997.

Anwar denies all the charges, saying he was framed by associates of Prime Minister Mahathir Mohamad who did not want him to succeed the premier.

Copyright 1998 Reuters Limited. All rights reserved.