The "Antonio Fire" was reported just after 2 p.m. Tuesday in the area of Glendora Ridge Road and Mount Baldy Road.
It started with a vehicle fire that spread to the brush, initially estimated at less than an acre. By 5 p.m. it had grown to 50 acres.
But the flames spread quickly amid hot, dry and windy conditions that are increasing the risk of fire all week throughout Southern California.
Firefighters with LA County and the Angeles National Forest responded with ground crews and aircraft, reporting they are "making solid progress containing the fire with hose-lays."