Four men were busted in the Bronx today for running a heroin mill out of a Sterling Avenue apartment that housed $1 million worth of drugs, authorities said.

The NYPD and the Drug Enforcement Agency said they seized seven kilos of heroin that had been packaged into 50,000 ready-to-use envelopes, or “glassines.”

The arrests were made in conjunction with an extensive investigation into a heroin trafficking ring operating in the Bronx.

Luis Lara, 28, who is believed by authorities to be the organization’s ringleader, was arrested as he was getting into a livery car with two other mill workers. Another man, Jose Polo, 28, was stopped with a backpack that contained approximately 3,000 glassines of heroin, officials said.

The four defendants are expected to be arraigned on criminal complaints later today. The top charge, criminal possession of a controlled substance in the first degree, carries a maximum 20-year sentence.