A “grenade” found at an East Harlem subway station Friday morning prompted an NYPD Bomb Squad investigation and shut down service during rush hour — though it turned out to just be plastic, police sources said.

The explosive – initially believed to be a real grenade – was discovered on the southbound No. 4 train tracks at Lexington Avenue and 125th Street around 8:45 a.m. Friday, the sources said.

As the Bomb Squad and Emergency Service Unit investigated, service for the Nos. 4 and 6 trains were halted between Manhattan and the Bronx in both directions.

No. 5 train service was suspended on the east side of Manhattan.

By around 10 a.m., the device was found to be a harmless piece of plastic, sources said.

The MTA also confirmed that service had been restored.