CD28 signaling does not stimulate p38 through the classic MAPK pathway. T cells purified from lymph nodes and spleen of wild-type or p38α^Y323F mice were stimulated with plate-bound α-CD3/CD28 for 5 minutes (MEK1/2 detection) or 30 minutes (MKK3/6). Phosphorylation of MEK1/2 and MKK3/6 was detected in whole cell lysates with the use of phosphospecific antibodies.