US Actor Ashton Kutcher has testified at a murder trial - telling a court that he went to pick up a woman for a date in 2001, but left when she did not answer the door.

He learned the next day she had been lying inside dead.

The actor said he had kept Ashley Ellerin, a 22-year-old fashion design student, waiting for hours on February 21st 2001 when he finally arrived to pick up the from her Hollywood home to go out for drinks.

"I knocked on the door and there was no answer," Kutcher said in the packed downtown Los Angeles courtroom during the trial of Michael Gargiulo, who is charged with killing Ms Ellerin and another woman.

"I knocked again, and once again, no answer. At this point I pretty well assumed she had left for the night, and that I was late, and she was upset."

Kutcher said he looked through the window and saw what he thought were wine stains on the floor, but did not think anything of it, and left.

He added: "I remember the next day after I heard about what happened, I went to the detectives and said 'my fingerprints are on the door'. I was freaking out."

Ms Ellerin was found dead by her roommate in the hallway outside her bathroom.

Defence Attorney Daniel Nardoni (left) and defendant Michael Gargiulo (right) listen to witness testimony by Ashton Kutcher during Gargiulo's murder trial charges in Los Angeles Superior Court | Image: Genaro Molina/AP/Press Association Images

Gargiulo, who prosecutors describe as a "serial sexual-thrill killer", is accused of entering the house and stabbing Ms Ellerin 47 times.

He has pleaded not guilty to two charges of murder and one of attempted murder in attacks between 2001 and 2008.

Gargiulo is charged separately with killing an 18-year-old Illinois woman, and is awaiting trial there.

Kutcher (41) is best known for his roles in US sitcoms 'That '70s Show' and 'Two And A Half Men'.

He is now a father of two and married to fellow actor Mila Kunis, after divorcing Demi Moore.

Called as a witness by prosecutors, the actor said he had recently reviewed the police report he had given the day after the killing, to refresh his memory about the incident 18 years ago.

He was briefly cross-examined and his answers were not challenged.

Ms Ellerin had been described as Kutcher's girlfriend by the media, but he said on Wednesday that she had been a casual acquaintance whom he had met a few months before at a party.

He added that he was going to take her out at the time because they were both single.

Kutcher said he had called Ms Ellerin in the afternoon from the set of 'That '70s Show' and had last spoken to her at around 8.30pm, when he was at a friend's house and she was just getting out of the shower.

He said he did not arrive at her house until around 10.45pm and called her in the interim four times.

The star could not reach her and he assumed it was bad phone service, adding: "Eighteen years ago, cell service wasn't it what it was today."

He said he did not leave messages every time he called, adding: "I was trying to take her on a date and didn't want to seem eager."