A total of 20 doping tests were conducted by the International Swimming Federation (FINA) at the opening event of its Champions Swim Series in Guangzhou in China.
In a statement, the worldwide governing body did not name any athletes tested but said it had carried out tests on a "number of race winners and athletes from a variety of nations".
Six were conducted for Erythropoiesis-stimulating agents – such as blood booster Erythropoietin – and for growth hormone releasing factors, according to FINA.
FINA is allowing athletes who have tested positive to participate at events in the series, launched in response to the International Swimming League (ISL) and which has promised greater prize money for competitors.
Controversial Chinese athlete Sun Yang, the Olympic 200 metres freestyle champion who served a three-month doping ban in 2014, competed in Guangzhou and won two gold medals.
Sun has also been accused of deliberately smashing his blood sample in a row with drugs testers earlier this year.
He received a warning for the offence, a decision which has been appealed by the World Anti-Doping Agency, which believes the sanction was too lenient.
The ISL, which has been at loggerheads with FINA since emerging on the scene, has vowed to ban swimmers who have failed drugs tests from its events.
It is unclear whether this will be legally enforceable as sports courts have consistently ruled against life bans.
The ISL is set to outsource drugs-testing at its events, while FINA will keep its anti-doping operation in-house for its Champions Swim Series competitions.
FINA has claimed it is "extremely satisfied by the positive reactions of champions invited to take part in the inaugural leg" of the series.
It comes despite the dispute with the ISL, which has accused FINA of copying its model.