3D/4 cells were infected with H1N1pdm, and cell lysates were prepared at various time points for SDS-PAGE and western blot analyses with specific anti-ERK1/2 and anti-JNK1/2 antibodies. Activated forms of ERK and JNK (phospho-ERK1/2 and phosphor-JNK1/2) were detected by anti-phospho-ERK1/2 and anti-phospho-JNK antibodies. As shown in Figure 3A , ERK1/2 was basally phosphorylated at a low level before infection, but further phosphorylated between 9 and 18 hrs and thereafter p.i.. Phosphorylation and activation of JNK1/2 appeared at 9 hrs and increased to the peak around 18 hrs p.i. (Fig. 3B) . Although both ERK1/2 and JNK1/2 were activated in response to H1N1pdm infection in swine macrophages, ERK1/2 remained active at basal level even before infection, so did JNK1/2 as shown in some of our experiments (Fig. 4B) . However, our data showed that basal level phosphorylation of both ERK1/2 and JNK1/2 remained unchanged in uninfected 3D/4 cells through the period of our infection. In addition to ERK1/2 and JNK1/2, we have also observed the phosphorylation and activation of p38 MAP kinase in H1N1pdm-infected cells (data not shown). 