Axenically cultured cells of Porphyridium cruentum were assayed for enzymes catalyzing the hydrolysis of lecithin. The sonicated cells showed virtually exclusive and complete conversion of 14C-choline-labelled lecithin to 14C-choline, with no significant formation of glycerylphosphorylcholine or phosphorylcholine. The enzymatic activity showed a sharp pH optimum at 7.0, and was strongly inhibited by chelating agents (two types), sulfhydryl-group binding reagents (three types), and surface-active agents (anionic and non-ionic). The inhibition from ethylenediamine tetraacetate (EDTA) was reversed by Ca2+ or Sr2+ but not by Mg2+ or Ba2+, and that from —SH binding reagents was reversed by dithiothreitol. Heavy metal ions (Zn2+, Cu2+, Ba2+, Co2+, Mn2+, Fe2+) inhibited the activity, Ca2+ caused stimulation, while Mg2+ and Sr2+ had little effect. The results indicate the occurrence of a pH and heavy-metal ion sensitive, Ca2+-(or Sr2+-) requiring phospholipase D in the red alga, and the involvement of sulfhydryl groups in the expression of its activity. The algal enzyme resembles those from higher plants in its Ca2+ requirement and sensitivity to EDTA and organomercurial sulfhydryl binding reagent, but differs markedly in its optimum pH and several other properties.