California Code, Civil Code - CIV § 1916-2

No person, company, association or corporation shall directly or indirectly take or receive in money, goods or things in action, or in any other manner whatsoever, any greater sum or any greater value for the loan or forbearance of money, goods or things in action than at the rate of twelve dollars upon one hundred dollars for one year; and in the computation of interest upon any bond, note, or other instrument or agreement, interest shall not be compounded, nor shall the interest thereon be construed to bear interest unless an agreement to that effect is clearly expressed in writing and signed by the party to be charged therewith. Any agreement or contract of any nature in conflict with the provisions of this section shall be null and void as to any agreement or stipulation therein contained to pay interest and no action at law to recover interest in any sum shall be maintained and the debt can not be declared due until the full period of time it was contracted for has elapsed.