Mid Yorkshire Hospital Trust introduced the new shifts for staff in the pathology department last Monday.
The changes mean they no longer work on set days and provide on-call cover.
But instead staff work 10 hour shifts which union representatives say are putting patients’ safety at risk.
Around 30 members of staff have been picketing outside the hospital since 2pm today (Tuesday).
Terry Cunliffe, Unite’s regional officer, said: “We are disputing the shift arrangements which are unsafe for both patients and staff.
“Staff are contracted to work 37 and a half hours but the shift patterns mean some of working in excess of that.
“Our members were told by the trust that if they did not agree to the changes their contacts would be terminated.
“The shift arrangements are not sustainable because there is not enough staff to support the structure and members are working extra.”
Union representatives met with senior managers at Mid Yorkshire Hospital Trust on Monday.
But Mr Cunliffe said talks broke down and the staff were left with no alternative but to strike.
He said: “The shift system is not safe and in my view to resolve it they need to go back the old system.
“If nothing changes then we are likely to do another day of action but when and what time has not been decided.”
The strike is expected is end at around 8pm.
Mid Yorkshire Hospital Trust’s director of operations Kevin Oxley said: “Over the last 12 months, we have been consulting with staff over the introduction of shift working within the blood sciences pathology team to ensure there is 24 hour cover on each hospital site.
“This will help us to provide a more efficient service to patients and will replace the existing on call arrangements.
“We are continuing to hold discussions with staff and trade union representatives about their concerns.
“Our primary concern during this four-hour stoppage has been to ensure continuity of patient care and safety whilst operating with reduced staffing levels and respecting the rights of individuals to choose either to participate in industrial action or to choose to work normally.”