The MLSA has warned that if no progress is made, a further two days of action are planned for 24 May and 25 May, with three further strikes planned for 31 May, 1 June and 2 June.
The chairperson of the MLSA Kevin O'Boyle said there is huge frustration and burn-out among medical scientists because of a severe recruitment and retention problem.
"Up to 20% of approved medical scientist posts are unfilled in hospitals," Mr O'Boyle said.
"Medical scientists carry out identical work to other colleagues in hospital laboratories, yet are paid on average 8% less," he added.
The MLSA says it has made every effort to avoid disruption to patients and fellow healthcare workers but has been left with no alternative.
The union represents more than 2,100 medical scientists employed in public voluntary hospitals, HSE hospitals, private hospitals and the Irish Blood Transfusion Service.
Selfish lot