Twenty-seven alkyl analogs of lysophospholipid were synthesized and their structureantimicrobial activity relationships were examined. These analogs differed in the structures of the long-chain alkyl moiety at position 1 and the β-N-substituted aminoethylphosphoryl moiety at position 3, and in the presence or absence of the 2-methoxy group of the glycerol moiety. Many of the alkyl lysophospholipids were found to possess antimicrobial activities much more potent than those of naturally occuring lysolecithin and lecithin against Tetrahymena pyriformis W and a variety of fungi, including human pathogens. The maximal activity was observed with 2-methyl-1-tetradecylglycero-3-phosphocholines. 1-Alkyl-2-methylglycero-3-phosphocholines with longer as well as shorter alkyl chains tended to have lower antimicrobial activity. Alkyl lysophospholipids with pyridinioethyl instead of the choline group showed potent antifungal activity comparable to alkyl glycerophosphocholines with the corresponding alkyl group but lower antiprotozoal activity. The tetradecyl congeners in these two classes of compounds showed potent inhibitory activity against Trichophyton species, comparable to that of clotrimazole. In contrast, alkyl lysophospholipids with an ethanolamine moiety in the polar head group showed decreased activity. Changing the molecular backbone from glycerol to 1, 3-propanediol had little effect upon the activity, and the resulting 1-alkyl-2-deoxyglycero-3-phosphocholines displayed antimicrobial properties similar to those of 1-alkyl-2-methylglycero-3-phosphocholines.