“It’s only been a few days, but we haven’t seen any deterioration on our performance metrics,” Ms. Lemkau said in an interview on Tuesday. She added that the company had also pulled ads from YouTube in the past week after reports showed other major advertisers like Verizon unintentionally appearing on videos promoting hate speech and terrorism. JPMorgan aims to restrict its ads on the platform to a “human-checked” list of 1,000 YouTube channels, which it expects to be able to do by the week of April 10, she said.

Much of the promise of online advertising hinges on the vast reach of the web, and the ability to reach people on niche sites at low prices. Index Exchange, an ad exchange, has estimated that the titles owned by the top 50 traditional media companies account for 5 percent or fewer of the trillions of ad impressions available for sale each day. Google’s display network alone includes more than two million websites. YouTube has more than three million ad-supported channels, according to the analytics company OpenSlate, which says the average $100,000 campaign on the platform runs on more than 7,000 channels.

If more advertisers follow JPMorgan’s lead and see similar results, it could hurt the operators of smaller sites that make up the so-called long tail of the internet, as well as the advertising technology companies that profit from funneling trillions of ad impressions from brands to consumers through systems that mimic a stock exchange, according to Eric Franchi, co-founder of the ad technology firm Undertone.

“If you charge a percentage of all of the ads that run through your platform, then the prospects can be pretty dim if all of a sudden your volume has been cut by 95 percent,” Mr. Franchi said. “So many of these companies, and some of them are public, tout the number of ads they deliver per second, per day. If you start seeing more marketers move in this direction, it will be pretty interesting. What are the metrics then that those companies start to report?”

JPMorgan started looking into preapproving sites, a strategy known as whitelisting, this month after The New York Times showed it an ad for Chase’s private client services on a site called Hillary 4 Prison. It was under a headline claiming that the actor Elijah Wood had revealed “the horrifying truth about the Satanic liberal perverts who run Hollywood.”