We recently found protein histidine phosphatase (PHP) in eukaryotes and identified ATP-citrate lyase (ACL) and the beta-subunit of G-proteins as its substrates. The aim of the present study was to get information on the significance of PHP for cellular function and viability. PHP was overexpressed by a viral vector in SH-SY5Y cells, a human neuroblastoma cell line, and in primary cultures of cortical neurons from embryonic (E19) rats. Furthermore, PHP was downregulated by siRNA in SH-SY5Y cells. We could demonstrate that overexpression of PHP decreased the phosphorylation state of ACL. Accordingly, the activity of ACL seemed to be reduced and subsequently, the viability of the cells was diminished. On the other hand, downregulation of PHP did not clearly influence phosphorylation and activity of ACL as well as viability of the cells. The results suggest that an increased activity of PHP impairs cellular function whereas downregulation of PHP does not.