MCP-1, CCR2–02, and crosslinked CCR2–02 Fab induce tyrosine phosphorylation of JAK2 kinase. Mono Mac 1 cells were stimulated for 1 min with 10 nM MCP-1, 15 nM CCR2–02 mAb, 1 μg/ml of CCR2–02 Fab plus 50 μg/ml of rabbit anti-mouse IgG Fab or 1 μg/ml of CCR2–02 Fab alone. Cell lysates were immunoprecipitated with anti-PTyr (PY20), electrophoresed, and transferred to nitrocellulose membranes. The Western blot was developed with an anti-JAK2 antibody. As a positive control, epidermal growth factor-stimulated A431 cell lysates were tested in Western blot with the same anti-JAK2 antibody. Arrow indicates the position of JAK2.