One of three major families of the mitogen-activated kinases (MAPK), p38 as well as JNK, has been shown to transduce extracellular stress stimuli into cellular responses by phospho-relay cascades. Among p38 families, p38alpha is a widely characterized isoform and the biological phenomena are explained by its kinase activity regulating functions of its downstream substrates. However, its specific contributions to each phenomenon are yet not fully elucidated. For better understanding of the role of MAPKs, especially p38alpha, we utilized newly established mouse fibroblast cell lines originated from a p38alpha null mouse, namely, a parental cell line without p38alpha gene locus, knockout of p38alpha (KOP), Zeosin-resistant (ZKOP), revertant of p38alpha (RKOP), and Exip revertant (EKOP). EKOP is smaller in size but grows faster than the others. Although comparable amounts of ERK and JNK are expressed in each cell line, ERK is highly phosphorylated in EKOP even in normal culture conditions. Serum stimulation after serum starvation led to ERK phosphorylation in RKOP and ZKOP, but not in EKOP as much. On the contrary, relative phosphorylation level of JNK to total JNK in response to UV was low in RKOP. And its phosphorylation as well as total JNK is slightly lower in EKOP. RKOP is less sensitive to UV irradiation as judged by the survival rate. Stress response upon UV or sorbitol stimuli, leading to mitogen activate protein kinase activated kinase 2 (MAPKAPK2) phosphorylation, was only observed in RKOP. Further experiments reveal that MAPKAPK2 expression is largely suppressed in ZKOP and EKOP. Its expression was recovered by re-introduction of p38alpha. The loss of MAPKAPK2 expression accompanied by the defect of p38alpha is confirmed in an embryonic extract prepared from p38alpha null mice. These data demonstrate that p38 signal pathway is regulated not only by phosphorylation but also by modulation of the expression of its component. Together, we have established cell lines that can be used in analyzing the functions of MAPKs, especially p38alpha, and show that p38 is indispensable for MAPKAPK2 expression.