The invention is directed to a masked chimeric antigen receptor, comprising: (a) a masking peptide; (b) one or more antigen-specific targeting domains; (c) an extracellular spacer domain; (d) a transmembrane domain; (e) at least one co-stimulatory domain; and (f) an intracellular signaling domain. The mCARs are activated upon cleavage of the masking peptide.