The effects that the bridging group has on nearest-neighbor recognition (NNR) in phospholipid membranes (i.e.? the thermodynamic preference for homodimer formation) have been examined using a homologous series of dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE). When 3,3＇-dithiodipropionyl (DTDP) was used as the exchangeable bridge, a statistical mixture of dimers was formed. In contrast, the use of a bridge that contained two additional methylene units resulted in a significant level of NNR; further extension of the bridge by two methylene units produced an additional increase in NNR. While cholesterol was found to induce significant NNR in bilayers made from lipid dimers having the DTDP moiety, its effect in membranes having longer bridging units was negligible. A simple model that accounts for these observations is presented, which is based on geometric and packing considerations. Experimental evidence in support of this model has been obtained from relative differences in the gel to liquid-crystalline phase transition temperatures and also from relative differences in fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), which have been measured in lipid membranes containing "short" and "long" bridges. Tighter packing in bilayers derived from phospholipid dimers having the DTDP bridge, together with the absence of nearest-neighbor recognition, points toward more cylindrically shaped phospholipids, and ones that are well-suited for model membrane studies. Possible biological implications of these findings are also briefly discussed.