PMA-induced leukotriene C₄ synthase (LTC₄S) phosphorylation was investigated over a period of 8 h in a monocytic cell line (THP-1). The level of LTC₄S phosphorylation was increased 3–5 fold over a 4 h period decreasing to basal levels after 8 h. This phosphorylation event was found to be specific to THP-1 cells as there was a lack of LTC₄S phosphorylation in both COS-7 and K-562 cells, and was also found to be dependent on the cellular confluency. In the presence of specific protein kinase C (PKC) inhibitors, a dose-dependent inhibition of the phosphorylation of LTC₄S became evident, an effect not seen with PKA and tyrosine kinase inhibitors. This represents the first direct demonstration of LTC₄S phosphorylation in whole cells.