Cal. Gov't Code § 22844

(a) Employees, annuitants, and family members who become eligible to enroll on or after January 1, 1985, in Part A and Part B of Medicare shall not be enrolled in a basic health benefit plan. If the employee, annuitant, or family member is enrolled in Part A and Part B of Medicare, he or she may enroll in a Medicare health benefit plan.
(b) Employees, annuitants, and family members enrolled in a prescription drug plan under Part D of Medicare shall not be enrolled in a board-approved health benefit plan. This subdivision does not apply to an individual enrolled in a board-approved or offered health benefit plan that provides a prescription drug plan or qualified prescription drug coverage under Part D of Medicare as part of its benefit design.
(c) This section does not apply to employees and family members that are specifically excluded from enrollment in a Medicare health benefit plan by federal law or federal regulation.
(d) The board shall not grant any further exemptions to this section after July 1, 2015.