Cal. Food & Agric. Code § 32761.5

The director shall test milk and milk products for the presence of drug residues. The director may adopt, by regulation, official drug residue testing methods and procedures for industry testing of raw milk from all bulk raw milk pickup tankers, prior to processing, for the presence of beta-lactam drugs and for other drug residues, as determined by the director. The director shall adopt, by regulation, official drug residue testing methods used for the detection of drug residue in farm bulk milk tanks, bulk milk pickup tankers, and milk and milk products in processing plants, wholesale outlets, and retail outlets.