Italian fraud squad detectives raided teams in the Giro d'Italia after Tuesday's 16th stage and seized medical files in an investigation into doping.
Cycling has been hit by a series of doping scandals
Eight riders had their hotel rooms searched in the early hours but police said they found no doping substances.
Seven other professional cyclists and seven professionals from other sports were also targeted.
The fraud squad has been investigating doping in sport, a criminal offence in Italy, since the start of the year.
Colonel Stefano Ortolani, of the paramilitary Carabiniere NAS anti-doping squad, would not reveal the names of the riders under investigation.
And he insisted the raids were not planned to coincide with the Giro.
"This was not a probe of the Giro," he said. "We went to the Giro because the cyclists were at the Giro and we are investigating them."
More than 100 homes, offices and hotel rooms were searched in the latest raids, and Ortolani said 138 people were under investigation for the illegal use and distribution of drugs in a variety of sports.
The Giro has been bedevilled by doping scandals in the past five years.
Former Tour de France winner Marco Pantani, who died of a cocaine overdose in February, was cleared last October of sporting fraud for a case of doping during the 1999 event.
Pantani was charged after high hematocrit levels were found in his blood during the 1999 Giro - an indicator but not proof that a rider may be using endurance-enhancing drugs.
The Italian, winner of both the Giro and Tour de France in 1998, served a six-month ban after insulin syringes were found in his hotel room during the 2001 Giro.
The same year the Fasso Bortolo team sacked Dario Frigo after illegal drugs, including the blood substitute Hemassit, were found in his room.
In 2002, Gilberto Simoni, the 2001 Giro winner, tested positive for cocaine, blaming the result on a visit to the dentist.
Stefano Garzelli, winner in 2000, also failed tests for the diuretic probencid that year.
Last year Russian rider Faat Zakirov, suspended from the 2002 Giro for doping, was handed a one-year ban plus a one-year suspended ban by the Court of Arbitration for Sport.
Zakirov's was the first case of a rider using Aranesp - otherwise known as darbepoietin, which mimics the effects of banned endurance enhancer EPO (erythropoietin).