Nearly 600 United Airlines employees face being fired after failing to comply with the firm's Covid-19 vaccination policy.
The vast majority of its 67,000 US staff have supplied proof of vaccination, which was required by Monday.
"This was an incredibly difficult decision," its bosses said in a memo to employees.
The Chicago-based airline set out its Covid requirements for staff in August.
Its US employees had to upload proof of vaccination, or the first of two jabs, by the deadline on Monday.
The 593 workers who have refused a coronavirus vaccine and have not applied for an exemption on religious or medical grounds now face losing their jobs.
A further 2,000 employees have requested an exemption to the policy.
It previously said it would put those who are exempt on temporary, unpaid leave from 2 October. But those plans were put on hold after a lawsuit was filed by six employees challenging the policy.
Elsewhere in the US, few airlines have introduced vaccine mandates for its staff. Delta Airlines, for example, has announced a $200 (£148) monthly health insurance surcharge for those who are not jabbed.
I definitely won't ever fly on United again. If you truly want to protest, protest with your $. Only spend your hard earned $ on businesses that believe in freedom. Also, I don't feel like being on an airplane that crashes because it's pilots had a heart attack while flying.