Ferrigno, 53, was presented with a badge, baton and Beretta 92FS handgun, E! Onlne reported.
"I am proud to be able to do my part as part of the team and to help these brave and dedicated deputy sheriffs in keeping Los Angeles County safe for the people who live, work and visit here," Ferrigno said in a statement.
Ferrigno will not be out on the street fighting crime, but will help recruit new classes of deputies and work with abused children.
The Los Angeles County Sheriff's Department Web site says reserve deputies put in 20 hours of service a month for an annual salary of $1.
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