Lonnie Franklin Jr.'s attorney entered pleas Monday to 10 murder counts and one charge of attempted murder. Franklin did not speak.
Prosecutors haven't decided if they'll seek the death penalty.
Franklin was arrested last month after authorities said DNA evidence linked the mechanic to the series of killings.
The slayings occurred from 1985 to 1988, and again from 2002 to 2007. The 14-year pause led to the killer's nickname of the "Grim Sleeper."
The victims were killed within a few miles of Franklin's South Los Angeles home.