Spanish doctor Eufemiano Fuentes, currently on trial for doping offences in cycling, has claimed he treated many sportspeople, among them footballers.
The trial of Fuentes and four co-defendants follows 'Operacion Puerto' in 2006, which saw Spanish police uncover evidence of widespread doping in professional cycling.
It began on Monday and is also expected to hear about illegal performance enhancement in other sports including tennis and football.
Fuentes spoke to reporters outside the court on Tuesday morning, and AS reporting him as saying: "In 2006 I worked with sportspeople of all types - footballers, cyclists and athletes.
"At that time, I did not work for a cycling team, but with individual sportspeople: an athlete, a footballer, a boxer..."
According to El Pais, Spanish police discovered 200 bags of blood that Fuentes and his colleagues had kept frozen or refrigerated in Madrid apartments. As doping was not illegal in Spain until 2006, the defendants are being charged with violating public health and safety laws.
Fuentes is an acknowledged expert on the use of EPO in sports, having published academic articles on the subject. In court, he is not expected to deny helping athletes but to argue that he did so without endangering their health or breaking any laws.
"If the sportsperson had very viscous blood, we removed some blood to avoid this danger," he said. "And we froze it.
"If then the sportsperson had a low haematocrit level or anaemia, we returned the blood for health reasons. Some removals took place in nearby hotels, because the sportsperson asked for privacy and not to meet with others at the laboratory."
Spanish authorities reportedly possess surveillance tapes showing a large number of as yet un-named sportspeople entering the building in which the bags of blood and related equipment were found.
Fuentes said the bags would have been tagged with a code and an alias, not the name of the person being treated.
"The bags carried a numerical code and sometimes also they asked to put an alias, which was always shorter than a name and surname," he said.
"The only people who knew the number and the alias on the bags were [Jose Luis] Merino, me, and the sportsperson. There was no confusion."
El Pais reported on Monday that the trial's "star witness" will be former cyclist Jesus Manzano, who told AS in 2004 that he had been involved in illegal performance enhancing activities and claimed to have met high-profile footballers at Fuentes' clinic.
Former US pro-cyclist Tyler Hamilton will also give evidence, judge Julia Patricia Santamaria has confirmed.
Fuentes did not want to discuss what his alleged former Spanish patient might say.
"I have no relationship with Jesus Manzano," he told reporters on Tuesday. Fuentes' fellow accused are his sister Yolanda Fuentes, Jose Iganacio Labarta, Manuel Saiz and Vicente Belda. All could face two years in prison.
The trial is scheduled to run until March 22.