we prove the Hyers-Ulam-Rassias stability of C ∗ -algebra homomorphisms and of generalized derivations on C ∗ -algebras for the following Cauchy-Jensen functional equation 2 f ( ( x + y ) / 2 + z ) = f ( x ) + f ( y ) + 2 f ( z ) , which was introduced and investigated by Baak (2006). The concept of Hyers-Ulam-Rassias stability originated from the stability theorem of Th. M. Rassias that appeared in (1978).