California Code, Health and Safety Code - HSC § 1373.18

Whenever any health care service plan, except a specialized health care service plan, negotiates and enters into a contract with providers to provide services at alternative rates of payment of the type described in Sections 10133 and 11512 of the Insurance Code, and enrollee copayments are to be based upon a percentage of the fee for services to be rendered, the amount of the enrollee copayment shall be calculated exclusively from the negotiated alternative rate for the service rendered. No health care service plan or provider, negotiating and entering into a contract pursuant to this section, shall charge or collect copayment amounts greater than those calculated in accordance with this section.

This section shall become operative on January 1, 1993.