Plant response to pathogens involves an intricate network of signal transduction pathways. Here, potato cell cultures were used to study signal transduction in response to elicitors fromPhytophthora infestans. Pretreatment of cells with Ser/Thr protein kinase inhibitors, EGTA, calmodulin antagonists or a channel blocker abolished the induction of two enzymes involved in defence responses, phenylalanine ammonia-lyase (PAL) and peroxidase. Phosphatase inhibitors caused an increase of these activities in the absence of elicitors. Hyphal cell wall components (HWC) from an incompatible race (HWC 0) produced a rapid and transient increment of histone phosphorylation, whereas induction by HWC from a compatible race (HWC C) was less pronounced and more sustained. As activities were calcium-dependent, a fraction enriched in calcium-dependent protein kinases (CDPKs) was obtained by DEAE chromatography. Fractions from HWC 0- and HWC C-treated cells presented higher kinase activity than that from untreated cells. Moreover, total activity was higher in the incompatible than in the compatible interaction. Activity was calcium-dependent, partially inhibited by calmodulin antagonists and able to phosphorylate syntide-2, a specific substrate of CDPKs. An in-gel kinase assay showed the presence of a band of approximately 50kDa whose activity was higher in HWC 0- than in HWC C-treated cells and was not detected in control extracts. This report presents evidences of the differential activation of CDPKs in response to elicitors from different races ofP. infestans, revealing that these protein kinases participate in the defence response to oomycete.