Several different vaccine strategies have been evaluated and combined in an attempt to amplify T-cell responses toward induction of antitumor immunity. The model tumor antigen used was carcinoembryonic antigen (CEA). While initial T-cell activation studies were conducted in conventional mice, combined vaccine strategy studies and antitumor studies were conducted in transgenic mice in which CEA is expressed in normal gastrointestinal tissue and CEA protein is found in sera. The studies reported here demonstrate: (a) A recombinant avipox (fowlpox, rF) vector expressing the signal 1 (CEA) and the B7-1 costimulatory molecule transgenes (designated rF-CEA/B7-1) is more potent in inducing CEA-specific T-cell responses than rF-CEA; one administration of recombinant fowlpox vector expressing CEA and three different costimulatory molecule transgenes (B7-1, ICAM-1, LFA-3, designated rF-CEA/TRICOM) was more potent in inducing CEA-specific T-cell responses than four vaccinations with rF-CEA or two vaccinations with rF-CEA/B7-1. Moreover, up to four vaccinations with rF-CEA/TRICOM induced greater CEA-specific T-cell responses with each vaccination. (b) A diversified prime and boost strategy using a prime with a recombinant vaccinia vector expressing CEA and the triad of costimulatory molecules (designated rV-CEA/TRICOM) and a boost with rF-CEA/TRICOM was more potent in inducing CEA-specific T-cell responses than the repeated use of rF-CEA/TRICOM alone. (c) The addition of granulocyte macrophage colony-stimulating factor (GM-CSF) to the rF-CEA or rF-CEA/TRICOM vaccinations via the simultaneous administration of a rF-GM-CSF vector enhanced CEA-specific T-cell responses. These strategies (TRICOM/diversified prime and boost/GM-CSF) were combined to treat CEA-expressing carcinoma liver metastases in CEA-transgenic mice; vaccination was initiated 14 days posttumor transplant. Antitumor effects in terms of survival and CD8(+) and CD4(+) responses specific for CEA were also observed in this CEA-transgenic mouse model. These studies demonstrate that the use of cytokines and diversified prime and boost regimens can be combined with the use of recombinant vectors expressing signal 1 and multiple costimulatory molecules to further amplify T-cell responses toward more effective vaccine strategies.