The human cDNA encoding a novel protein serine/threonine kinase most closely related to p70 S6 kinase was isolated from the human erythroleukemia cDNA library and termed p70 S 6 K β . p70 S 6 K β has 67% amino acid identity in overall sequence with human p70 S 6 K , and the potential phosphorylation sites of p70 S 6 K are conserved in p70 S 6 K β . Northern blot analysis identified two major transcripts of p70 S 6 K β that are ubiquitously expressed in human adult tissues. Similar to p70 S 6 K , p70 S 6 K β was activated by serum stimulation, and the serum-induced activation was inhibited by wortmannin and rapamycin. These findings suggest that p70 S 6 K β is an isoform of p70 S 6 K with similar regulatory mechanisms.