Cal. Pub. Cont. Code § 10167

All bids shall be presented under sealed cover and accompanied by one of the following forms of bidder’s security: cash, a cashier’s check, certified check, or a bidder’s bond executed by an admitted surety insurer, made payable to the director of the department under which the work is to be performed. The security shall be in an amount equal to at least 10 percent of the amount bid. A bid shall not be considered unless one of the forms of bidder’s security is enclosed with it.