Inositol phospholipid turnover is part of a signal transduction mechanism which mobilize intracellular calcium and activate a calcium- and phospholipid-dependent protein kinase, protein kinase C. Phosphatidylinositol turnover has recently been implicated in the regulation of cell proliferation and transformation. Its role in differentiation has now been investigated using LAN-1 cells, a human neuroblastoma cell line which can be induced to differentiate along the neuronal pathway by RA. Treatment of LAN-1 cells with RA was followed by a rapid decrease of inositol phospholipid metabolism, as determined by isotopic methodology employing myo-[1,2-3H] inositol or [1(3)-3H] glycerol. Analysis of labelled phosphatidylinositol metabolites from prelabelled cells indicated a rapid decrease of inositol (1,4,5)trisphosphate and (1,2)diacylglycerol within 1 min. of induction of LAN-1 cell differentiation. These findings suggest that inositol phospholipid-derived metabolites (i.e. diacylglycerol and inositol trisphosphate) may be part of the mechanism by which certain RA signals are transduced, playing a key role in control of neuroblastoma cell differentiation.