Amazon.com is tapping suppliers to make a foray into private-label sportswear.

Amazon.com is tapping some of the biggest athletic apparel suppliers to make a foray into private-label sportswear, according to people familiar with the matter, setting the stage for further upheaval in an already tumultuous industry.

Makalot Industrial, a Taiwanese vendor that produces clothing for Gap, Uniqlo and Kohl’s Corporation, is making apparel for the Amazon line, a person with knowledge of the arrangement said. Eclat Textile, another Taiwanese supplier, is contributing to the effort as well — a relationship first noted by SinoPac Securities Corporation analyst Silvia Chiu.

The project is new and longterm contracts have not been signed yet, according to people involved. The manufacturers are producing small amounts of products for Amazon as part of a trial, said the people, who asked not to be identified because the effort is not being promoted yet.

Amazon has previously ventured into private-label fashion, offering office clothing, jackets and dresses under names such as Goodthreads and Paris Sunday. But pushing into activewear would bring fresh competition to some of the world’s biggest athletic brands.

Eclat’s involvement is especially noteworthy as it makes clothing for Nike, Lululemon Athletica and Under Armour and has key expertise in making high-performance sportswear.

Shares of Lululemon fell as much as 4.9%, to $57.55 after Bloomberg reported on Amazon’s sportswear efforts on Friday. Under Armour declined as much as 2.8%. Nike also slipped, though its shares recovered, closing up 0.3% at $50.98.

The move comes as unwelcome news for activewear firms struggling to stand out in a sea of competition and discounts. In September, Nike said it expected sales to fall again this quarter in North America.