Marek's disease (MD) vaccine does not provide sterilizing immunity that prevents subsequent MD virus (MDV) replication and shedding in vaccinated birds. It is hypothesized that cell-mediated immunity is critical to control the virus replication in chickens because MDV exists in cell-associated forms in the host. To improve the MD vaccine efficacy, particularly cell-mediated immunity, we constructed recombinant v301B/1-IL-15, an MDV serotype 2 vaccine strain 301B/1 expressing chicken interleukin-15 (IL-15), a cytokine which promotes T-cell proliferation and enhances T-cell responses. We examined the vaccine efficacy of v301B/1-IL-15 given as a bivalent MD vaccine in combination with turkey herpesvirus (HVT) against a very virulent MDV challenge. The expression of IL-15 did not interfere with virus stability and the growth of recombinant v301B/1-IL-15. However, the protective efficacy of v301B/1-IL-15 was not significantly different from that of v301B/1, the parental virus used to construct v301B/1-IL-15. Shedding of challenge virus was slightly reduced at Day 21 (16 days postchallenge) in the v301B/1-IL-15 plus HVT vaccinated group, with no statistically significant difference to that of the v301B/1 plus HVT vaccinated group, and thymus atrophy was observed to be less severe in the v301B/1-IL-15 plus HVT vaccinated group. Overall, the protection of v301B/1-IL-15 was not differentiable from v301B/1 against very virulent MDV challenge, but there is no interference with bivalent MD vaccine efficacy.