Indirubin-3'-monoxime is a derivative of the bis-indole alkaloid indirubin, an active ingredient of a traditional Chinese medical preparation that exhibits anti-inflammatory and anti-leukemic activities. Indirubin-3'-monoxime is mainly recognized as an inhibitor of cyclin-dependent kinases (CDKs) and glycogen synthase kinase-3. It inhibits proliferation of cultured cells, mainly through arresting the cells in the G1/S or G2/M phase of the cell cycle. Here, we report that indirubin-3'-monoxime is able to inhibit proliferation of NIH/3T3 cells by specifically inhibiting autophosphorylation of fibroblast growth factor receptor 1 (FGFR1), blocking in this way the receptor-mediated cell signaling. Indirubin-3'-monoxime inhibits the activity of FGFR1 at a concentration lower than that required for inhibition of phosphorylation of CDK2 and retinoblastoma protein and cell proliferation stimulated by fetal calf serum. The ability of indirubin-3'-monoxime to inhibit FGFR1 signaling was similar to that of the FGFR1 inhibitor SU5402. In addition, we found that indirubin-3'-monoxime activates long-term p38 mitogen-activated protein kinase activity, which stimulates extracellular signal-regulated kinase 1/2 in a way unrelated to the activity of FGFR1. Furthermore, we show that indirubin-3'-monoxime can inhibit proliferation of the myeloid leukemia cell line KG-1a through inhibition of the activity of the FGFR1 tyrosine kinase. The data presented here demonstrate previously unknown activities of indirubin-3'-monoxime that may have clinical implications.