California Code, Public Contract Code - PCC § 10765

(a) 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 trustees. 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.

(b) A bid described in subdivision (a) may be submitted electronically in accordance with Section 1601, but only if the bidder submits the bidder's security required by subdivision (a) within 24 hours after the opening of bids.