The in vitro antineoplastic activity of many phosphorus-containing (e.g., phosphocholines) and non-phosphorus-containing (e.g., quaternary ammonium salts) ether lipids has been evaluated in the HL-60 promyelocytic cell line. These compounds are analogues of ET-18-OMe (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine). Structural modification of 1-(alkylamido)-,-(alkylthio)-, and -(alkyloxy)propyl backbones has provided further insight into the structure-activity relationships of these lipids. In this study, along saturated C-1 chain and a three-carbon backbone with a single short C-2 substituent were preferred. At the positively charged nitrogen of phosphocholines, fewer than three substituents caused a significant loss of activity, and substituents larger than methyl decreased activity slightly. In the nonphosphorus compounds, many nitrogen heterocycles and also a sulfonium moiety were incorporated without changing the degree of activity; however, a thiazolium group decreased activity. The most active compound, 29 [N-[3-(hexadecyloxy)-2-methoxypropyl]-3-(hydroxymethyl)pyridinium bromide], was approximately twice as active as the reference standard, ET-18-OMe, in a trypan blue dye exclusion assay.