Termination of synaptic glutamate transmission depends on rapid removal of glutamate by neuronal and glial high-affinity transporters. Molecular biological and pharmacological studies have demonstrated that at least five subtypes of Na+-dependent mammalian glutamate transporters exist. Our study demonstrates that Y-79 human retinoblastoma cells express a single Na+-dependent glutamate uptake system with a Km of 1.7 ± 0.42 μM that is inhibited by dihydrokainate and DL-threo-β-hydroxyaspartate (IC50 = 0.29 ± 0.17 μM and 2.0 ± 0.43 μM, respectively). The protein kinase C activator phorbol 12-myristate 13-acetate caused a concentration-dependent inhibition of glutamate uptake (IC50 = 0.56 ± 0.05 nM), but did not affect Na+-dependent glycine uptake significantly. This inhibition of glutamate uptake resulted from a fivefold decrease in the transporter's affinity for glutamate, without significantly altering the Vmax. 4α-Phorbol 12,13-didecanoate, a phorbol ester that does not activate protein kinase C, did not alter glutamate uptake significantly. The phorbol 12-myristate 13-acetate-induced inhibition of glutamate uptake was reversed by preincubation with staurosporine. The biophysical and pharmacological profile of the human glutamate transporter expressed by the Y-79 cell line indicates that it belongs to the dihydrokainate-sensitive EAAT2/GLT-1 subtype. This conclusion was confirmed by western blot analysis. Protein kinase C modulation of glutamate transporter activity may represent a mechanism to modulate extracellular glutamate and shape postsynaptic responses.