In addition to TCR signaling, the activation and proliferation of naive T cells require CD28-mediated co-stimulation. Once engaged, CD28 is phosphorylated and can then activate signaling pathways by recruiting molecules to its YMNM motif and two PxxP motifs. In this study, we analyzed the relationship between tyrosine phosphorylation and the co-stimulatory function of CD28 in murine primary CD4(+) T cells. Tyrosine phosphorylation is decreased in CD28 where the N-terminal PxxP motif is mutated (nPA). In cells expressing nPA, activation of Akt and functional co-stimulation were decreased. In contrast, where the C-terminal PxxP motif is mutated, tyrosine phosphorylation and activation of the ERK, Akt and NF-κB were intact, but proliferation and IL-2 production were decreased. Using the Y(189) to F mutant, we also demonstrated that in naive CD4(+) T cells, tyrosine at position 189 in the YMNM motif is critical for both tyrosine phosphorylation and the functional co-stimulatory effects of CD28. This mutation did not affect unfractionated T-cell populations. Overall, our data suggest that CD28 signaling uses tyrosine phosphorylation-dependent and phosphorylation-independent pathways.