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Serum contains a factor that co-purifies with albumin and causes neurite retraction in PC12 cells, inhibits the proliferation of tumor cells in vitro, and activates the phosphatidylinositol/Ca2+ second messenger system in Xenopus oocytes and other cells. The activity of serum albumin depends on several lysophospholipids bound to albumin. Thin layer chromatographic analysis of the lipids extracted by methanol from serum albumin revealed over a dozen components, several of which evoked oscillatory currents in oocytes. In contrast to serum albumin, most of these lipids were absent in plasma, which lacks the biological activity. The most abundant naturally occurring active component was identified as stearoyl-lysophosphatidic acid. Synthetically prepared lysophosphatidates reproduced the biological activities of the natural serum factor. Adding synthetic lysophosphatidates to inactive fatty acid-free albumin restored …