Suppression of cell-mediated immunity (CMI) is well-documented during and after measles. This immunosuppression is suggested to result from decreased production of interleukin-12 (IL-12), a key interleukin for CMI. In an attempt to clearly discern the role of IL-12 in measles-induced immunosuppression, a measles virus (MV) that expresses biologically active human IL-12 was generated. This was achieved by inserting the coding sequences of the two subunits (p35 and p40) of human IL-12 separated by an internal ribosome entry site in an additional transcription unit between the H and the L genes of MV. Although the IL-12-expressing MV grew slightly slower than the normal MV, it stably maintained the inserted sequences (3.2 kb) and uniformly expressed the foreign genes after 10 passages in cell culture. These findings suggest that MV is a well-suited vector for delivery of proteins of immunogenic and therapeutic importance.