The incorporation of ethanolamine-1,2-14C into phospholipid and the decarboxylation of phosphatidyl-serine-1,2-14C by etiolated pea seedling "microsomal suspensions" have been investigated. Optimum incorporation occurred with phosphate buffer, pH 8.5, in the presence of 2.9 × 10−3 M CaCl2. Toluene, phospholipase D, and reduced glutathione stimulated ethanolamine-1,2-14C incorporation. Higher concentrations of CaCl2, imidazole buffer, and Tris buffer were inhibitory. Other divalent cations, ATP, CTP, and CMP were without effect. L-Serine and choline were also incorporated into phospholipids by pea "microsomal suspensions" and competitively inhibited the incorporation of ethanolamine-1,2-14C. Inhibition was also observed with D-serine and glycerol, but not with methanol, ethanol, propane-1-ol, and propane-2-ol. The formation of 14CO2 by the decarboxylation of serine-U-14C by "microsomal suspensions" was also stimulated by Ca2+ and inhibited by ethanolamine. The experimental results suggest that the incorporation of ethanolamine and the decarboxylation of serine in plants proceeds by essentially the same Ca2+-requiring mechanisms reported for animals.