The HSE's warning patients to expect 'significant disruption' in hospitals today, due to industrial action.
It involves 2,100 medical scientists, who are picketing over pay and career development issues.
They're refusing to carry out routine lab services from 8am to 8pm today.
Hospitals across the West including Mayo University Hospital will be affected.
They mainly work in public hospitals - and Liam Woods, the HSE's director of acute operations, says the action will affect services.