More than 70 medical secretaries from Nipsa took strike action at Derry's Altnagelvin Hospital.
The union announced the move last week in a dispute over workload. Further steps planned include a work-to-rule involving an overtime ban and non-cover for vacant posts.
The strike followed a ballot and came as Nipsa's classroom assistants continue two days of industrial action each week. The public service union said it planned stoppages in the hospital in October but agreed to suspend the action to provide the Western Health & Social Care Trust an opportunity to bring forward proposals to alleviate a staffing crisis.
Last month, a 10-day strike by classroom assistants shut more than half of special schools. Some schools organised a rota system last week to accommodate high risk pupils during the walk-out over pay and conditions.
Assistants apologised to parents but said they were determined to continue with the action. Talks with employers at the North's Labour Relations Agency failed.
PA