THE mitogen-activated protein (MAP) kinases Erk-1 and Erk-2 are proline-directed kinases that are themselves activated through concomitant phosphorylation of tyrosine and threonine residues^1–4. The kinase p54 (M_r 54,000), which was first isolated from cycloheximide-treated rats, is proline-directed like Erks-1/2, and requires both Tyr and Ser/Thr phosphorylation^3,5,6 for activity. p54 is, however, distinct from Erks-1/2 in its substrate specificity, being unable to phosphon late pp90^rsk but more active in phosphor-ylating the c-Jun transactivation domain^5,7,8. Molecular cloning of p54 reveals a unique subfamily of extracellularly regulated kinases. Although they are 40–45% identical in sequence to Erks-1/2, unlike Erks-1/2 the p54s are only poorly activated in most cells by mitogens or phorbol esters. However, p54s are the principal c-Jun N-terminal kinases activated by cellular stress and tumour necrosis factor (TNF)-α, hence they are designated stress-activated protein kinases, or SAPKs. SAPKs are also activated by sphingo-myelinase, which elicits a subset of cellular responses to TNF-α (ref. 9). SAPKs therefore define a new TNF-α and stress-activated signalling pathway, possibly initiated by sphingomyelin-based second messengers, which regulates the activity of c-Jun.