Five additional strike days are planned if a resolution is not found.
The HSE's warning patients to expect 'significant disruption' in hospitals today, due to industrial action.
The Medical Laboratory Scientists Association (MLSA), which represents more than 2,100 medical scientists, has said the dispute centres on a decades-long demand for pay parity with colleagues in laboratories who are doing the same work.
The union has said medical scientists are paid on average 8% less than colleagues in hospital laboratories who are doing the same work.
They're refusing to carry out routine lab services from 8am to 8pm today.
The MLSA said it has made every effort to avoid today’s disruption to patients and fellow healthcare workers, but has been left with no alternative.
The action follows many rounds of unsuccessful talks with the HSE, Department of Health, Department of Public Expenditure and Reform and the Public Service Agreement Group.
In a ballot of MLSA members last November 98% had voted in favour of taking the action.
They mainly work in public hospitals - and Liam Woods, the HSE's director of acute operations, says the action will affect services.
If no progress is made a further two days of action are planned for May 24th and 25th.
Three further days of action are planned for May 31st, June 1st and June 2nd.
Terry Casey is the general secretary of the Medical Laboratory Scientists Association.
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