In Ewing's sarcomas, chromosomal translocations cause the N-terminal domain of the EWS (Ewing's sarcoma protein) to fuse with the DNA-binding domains of the Ets (E26 transformation-specific) family of transcription factors. Here we show that EWS and EWS-Fli1 (Friend leukaemia virus integration 1), the fusion most frequently found in Ewing's sarcomas, become phosphorylated at Thr(79) in response to either mitogens or DNA-damaging agents. The much weaker mitogen-induced phosphorylation of EWS is catalysed by the MAPKs (mitogen-activated protein kinases) ERK1 (extracellular signal-regulated kinase 1) and ERK2, whereas the much stronger phosphorylation of EWS induced by the DNA alkylating agent MMS (methyl methanesulphonate) can be catalysed by JNK (c-Jun N-terminal kinase) and at least one other protein kinase distinct from ERK1/ERK2. In contrast, the phosphorylation of EWS-Fli1 induced by MMS was largely mediated by p38alpha/p38beta MAPKs. MMS induced a much stronger phosphorylation of EWS-Fli1 than EWS in heterodimers comprising both proteins.