SAN FRANCISCO — A key witness in cyclist Tammy Thomas’ doping trial testified Wednesday that an illicit steroid lab in Illinois made little money because the potent performance-enhancers could be bought and taken in such small quantities.

“Here’s the thing: the stuff was so strong, man,” said Kelcey Dalton, who helped market the substances developed and manufactured by Patrick Arnold, her boyfriend at the time.

Arnold, a chemist, invented some of the steroids at the heart of a drugs and sports scandal in which Thomas is the first person being tried.

Dalton, testifying on the third day of Thomas’ trial, said Arnold told her that athletes and trainers were reporting in the early 2000s they would use “really minute doses” of the steroids because doing so would minimize side effects, and “because they were so strong.”

Holding her thumb and forefinger three inches apart to represent a small vial, Dalton said such a container would fetch $10-$20. “Marion Jones split it with C.J. Hunter and it lasted six months,” Dalton said, referring to the track and field stars later disgraced for doping.

The sums of money Arnold was making were “very low,” she said. “I think we should have charged more.”

During her testimony, Dalton also said she heard Bay Area Laboratory Co-Operative founder Victor Conte say he had provided steroids to Jones, imprisoned earlier this month for lying to investigators about doping and her role in a check-fraud scheme; and to home run king Barry Bonds.

In an e-mail to news media organizations, Conte firmly denied he had supplied Bonds, who faces perjury charges stemming from the doping scandal.

“Kelcey Dalton is most certainly mistaken by saying that I ever told her that I gave any type of performance enhancing drugs to Barry Bonds,” Conte wrote. “I may possibly have told her that I was working with Barry and had him on a comprehensive nutrition program, but never did I say to her that Barry Bonds used drugs.”

Arnold didn’t mention Bonds a day earlier when he listed athletes he believed were being supplied his steroid products. Thomas’ case is being closely watched by Bonds observers looking for hints at how a potential trial for the former Giants slugger might play out.

Bonds attorney Allen Ruby declined to comment Wednesday.

Prosecutors say Thomas lied to a grand jury about what performance-enhancing substances she bought from Arnold, and whether she had ever taken anabolic steroids. She is charged with making false statements to a grand jury and obstructing justice.

Dalton testified Wednesday she believed Thomas knowingly sought what she thought were undetectable steroids from Arnold. But under cross-examination by Thomas attorney Ethan Balogh, Dalton also said she did not believe Thomas thought she was breaking the law, because the substances were not then banned under federal law.

Balogh has argued that Thomas was technically telling the truth to the grand jury when she denied using steroids.