Subway workers in Buenos Aires went on strike late Tuesday over rail cars from Spain used in the Argentine capital's metro system that allegedly contain asbestos.
At the center of the claims are cars bought second-hand from the Madrid metro in 2011, when President Mauricio Macri was the city's mayor.
The cars were withdrawn from service last month as a preventive measure while authorities investigate.
The strike affected Buenos Aires' five metro lines for a four-hour period starting at 11:00 pm (0200 GMT).
Metro de Madrid, which runs the Spanish capital's service, opened an investigation to determine if there were irregularities in the sale of the cars to Argentina.
The company sold a total of 36 metro cars to Subterraneos de Buenos Aires in a deal worth around five million euros ($6 million).
Asbestos, used as a fire retardant, has been banned in Argentina since 2001 because of its carcinogenic properties. It was banned in Spain the same year.
"We have requested certificates from the train manufacturers to determine whether or not there is asbestos," a spokesman for the state-owned company told AFP earlier this month.
Claudio Dellacarbonara, spokesman for the metro workers' union, said that in Spain "there have already been cases of workers who have become ill because of this situation."
He said his union had managed to get CAF 5000-model cars taken out of service, but it had now emerged that asbestos had been found in at least one CAF 6000 car in Spain, similar to a model that is used on Line B of Buenos Aires' metro.