Section 1608 of the California Civil Code has been on the books since 1872.  It provides:

"If any part of a single consideration for one or more objects, or of several considerations for a single object, is unlawful, the entire contract is void."

What does this mean for contracts involving marijuana?  The Controlled Substances Act makes it illegal under federal law to manufacture, distribute, or dispense marijuana (Cannabis).  21 U.S.C. Sec. 801 et seq.  The situation is different in California because the voters have largely legalized Cannabis related businesses through the passage of Proposition 215 (The Compassionate Use Act of 1996) and Proposition 64 (the Control, Regulate, and Tax Adult Use of Marijuana Act of 2016).  In addition the Medicinal and Adult-Use Cannabis Regulation and Safety Act (MAUCRSA), Cal. Bus. & Prof. Code Sec. 26000 et seq. impose a comprehensive system of control and regulation of the cultivation, distribution, transportation, storage, manufacturing, processing and sale of medicinal and adult-use Cannabis.

The Civil Code does not specify whether the unlawfulness must be under California law.  At one time, the possession, use and transport of Cannabis was illegal under California law.  In Bovard v. American Horse Enterprises, Inc., 201 Cal. App. 3d 832 (1988), the Court of Appeal considered the enforceability of a contract for the sale of a business engaged in the manufacture of marijuana paraphernalia:

"Moreover, it is immaterial that the business conducted by American Horse Enterprises was not expressly prohibited by law when Bovard and Ralph made their agreement since both parties knew that the corporation's products would be used primarily for purposes which were expressly illegal.  We conclude the trial court correctly declared the contract contrary to the policy of express law and therefore illegal and void."

Now that the legal status of marijuana has changed, the question will unlawfulness under federal law be sufficient to void contracts?  

Update:  Phil Schroeder at Buchalter has pointed out Civil Code Section 1550.5(b) which was added to the Civil Code effective January 1, 2018, Stats. 2017, Ch. 530 (AB 1159).  That statute provides: 

"Notwithstanding any law, including, but not limited to, Sections 1550, 1667, and 1668 and federal law, commercial activity relating to medicinal cannabis or adult-use cannabis conducted in compliance with California law and any applicable local standards, requirements, and regulations shall be deemed to be all of the following:

(1) A lawful object of a contract.
(2) Not contrary to, an express provision of law, any policy of express law, or good morals.
(3) Not against public policy."

This raises the interesting question of whether California's declaration that conduct that is unlawful under federal law is "lawful" would allow a court to provide relief.  As a general matter, judges should not be countenancing illegal activities.  A federal court in particular may have concerns about opening its doors to parties engaged in activities that remain illegal under federal law, regardless of their deemed lawfulness under California law.