MADRID - Spanish doctor Eufemiano Fuentes, on trial for allegedly running a doping ring in cycling, said on Wednesday he may be willing to cooperate with anti-doping authorities and name the clients whose blood was found in bags seized by police.

"If they (anti-doping authorities) believe that I am useful and they ask me (to cooperate) I would consider it and I would be ready," Fuentes told reporters attending the trial in Madrid.

"What I don't know is if what I could contribute would be worth anything to them or not," he said, adding that what he envisaged was a "mutual collaboration".

"If they want my involvement to include the list (of my clients) they would have it."

Fuentes and four other defendants, including his sister Yolanda, have been appearing in court since late January, almost seven years after anabolic steroids, transfusion equipment and blood bags were seized as part of a investigation code-named "Operation Puerto".

The proceedings have attracted international scrutiny because anti-doping authorities are hopeful it will finally lead to evidence of wrongdoing by athletes in sports other than cycling being made available.

Fuentes told the court in his opening testimony that as well as a host of professional cyclists he also had clients in sports including soccer, tennis, athletics and boxing.

A World Anti-Doping Agency (WADA) request for access to the blood bags has been repeatedly denied by the Spanish authorities, and the agency, which is taking part in the trial, awaits the judge's ruling on their latest petition.

The accused have a final opportunity to speak to the court on April 2 before sentencing begins.

As Spain's current anti-doping legislation was not in force in 2006 when the police raids took place, the five are being tried for violating public health regulations. The prosecutor has asked for jail sentences of two years.