A leading ad exchange has blacklisted Breitbart News, which until recently was run by one of US President-elect Donald Trump's closest advisers.
AppNexus said it would no longer allow Breitbart to sell ad space via its platform, after determining that the site had broken its code on hate speech and incitement to violence.
Breitbart responded saying it "has always and continues to condemn racism and bigotry in any form".
AppNexus has not given examples.
But a spokesman said a "human audit" of Breitbart had flagged several articles that had caused it concern because of the language they had featured.
"We use a number of third-party standards to determine what is and isn't hate speech, and if we detect a pattern of speech that could incite violence or discrimination against a minority group, we determine that to be non-compliant and we simply won't serve ads against it," AppNexus's spokesman Joshua Zeitz told the BBC.
"I'm not going to put the examples out there because I'm not going to engage in a tit-for-tat on what is compliant."
The Bloomberg news agency was first to report the development.
It noted that AppNexus' investors included Microsoft, News Corp and Sir Martin Sorrell's WPP.
The move follows an earlier ban by Twitter of one of Breitbart's most prominent writers, after claims he had incited abuse against an actress.