Yellow marks under the arms on white and light tees are one of the most common returns. They are usually caused by a build-up of sweat, body oils and the residues of deodorants or antiperspirants in the fabric, which wash out only partly and then discolour over time. The stain is not usually a sign that the fabric was made badly, but your fabric and finish can make it more or less likely, so it is worth testing.

Separate causes before you judge

Ask where and when the yellowing appears. If it shows only under the arms, comes after wear and is worse on tees worn with deodorant, it points to build-up. If it appears in storage on unworn garments, it may be a different issue, such as yellowing from finishes or packaging, which is covered in our post on tees yellowing before sale. If it appears evenly across the fabric, check the fabric and optical brightener use.

What fabric tests can show

ISO 105-E04 tests colour fastness to perspiration, which tells you whether dyes change or stain other fabrics when wetted with test perspiration solutions. It is mainly useful for coloured tees. It does not copy long-term deodorant build-up, so no single standard test result will promise that a white tee will never yellow. Ask your lab whether it can run a wear-simulation or a customised test using the deodorant types your customers use, and treat the result as a comparison between fabrics, not a guarantee.

Fabric and construction choices

Fabric weight, fibre mix and finishes can all affect how much residue is held in the underarm area. Some brands add underarm sweat pads, use a looser fit under the arm, or choose a fabric that dries faster. Compare options in a trial: have several fabrics worn and washed, then compare the underarm area under good light.

What the care label and product page can say

The FTC Care Labeling Rule, 16 CFR Part 423, requires care instructions supported by a reasonable basis. Wash tests with a named procedure such as ISO 6330 will show how your fabric holds up with your label instructions. On the product page, you can add practical advice such as washing soon after wear and letting deodorant dry before dressing, without promising the stain will never appear.

Customer service

Agree a clear approach to returns. If you find that your fabric yellows more than others in trial, treat it as a product issue and change the fabric.