Software firm Salesforce.com is being sued by 50 women who allege it aided gangs that trafficked them for sex via the Backpage website.
Backpage used the firm's software to keep its classified ads site running.
The site was closed after its founders were arrested and accused of facilitating prostitution and money laundering.
Salesforce said it took the allegations "seriously" and was committed to "ethical" use of its products.
The lawsuit claims Salesforce.com did not properly vet Backpage, which was seized and shut down last year by US law enforcement.
Court documents accuse Salesforce.com of working closely with Backpage on the database that underlays the business. Salesforce.com is best known for making software that firms use to monitor and manage sales relationships with customers.
The connection between the two firms, the lawsuit alleges, continued even after 2013 when Backpage was first accused of being a hub for the trafficking of women.
An investigation by Reuters suggested that as many as one in 20 of the relevant posts on some regional Backpage sites could have involved slavery victims.
The documents filed to the court also claim that Salesforce.com gave help to Backpage so it could reach new customers which, in this case, were gangs looking to sexually exploit women.
In a statement, Salesforce.com said it could not comment on pending litigation.
It added: "We are deeply committed to the ethical and humane use of our products and take these allegations seriously."