A long-standing beef between neighbors turned bloody, leaving one person slashed, another hit in the face with bleach and a mother and son among three people arrested when it was all over, residents and cops said.

The feud boiled over in the lobby of the NYCHA East River Houses building at 410 East 105th Street at about 7:30 a.m. Saturday, police said.

Valena Humphrey, 27, got into an argument with Midaglia Santiago, 52, and her son, Anthony Parra, 25, over a previously unknown incident, an NYPD spokeswoman said.

Humphrey allegedly threw bleach into Parra’s face while he entered the lobby, prompting Parra and his mom to retaliate by slashing Humphrey, cops said.

All three combatants were charged with assault and criminal possession of a weapon, a knife, police said. Humphrey suffered arm, neck and leg lacerations in the fracas.

The trio was taken to New York- Presbyterian/Weill Cornell Medical Center.

The NYPD did not know what fueled the fighting.