To better understand the antigenic requirements for cross-presentation, we compared the in vivo efficiency of presentation of cell-associated vs soluble OVA with the OT-I (CD8) and OT-II (CD4) TCR transgenic lines. Cross-presentation of cell-associated OVA was very efficient, requiring as little as 21 ng of OVA to activate OT-II cells and 100-fold less to activate OT-I cells. In contrast, soluble OVA was presented inefficiently, requiring at least 10,000 ng OVA for activation of either T cell subset. Thus, cell-associated OVA was presented 500-fold more efficiently than soluble OVA to CD4 T cells and 50,000-fold more efficiently to CD8 T cells. These data, which represent the first quantitative in vivo analysis of cross-presentation, show that cell-associated OVA is very efficiently presented via the class I pathway.