Tracking of single marker-particles is a powerful method for studying the dynamics of cell surfaces. However, only a few types of particles are available for tracking and applications of these particles are limited. We propose to develop and characterize sensitive, specific fluorescent marker immunoliposomes useful for particle tracking and other optical techniques. We will explore phospholipid compositions and fluorescent labels that give the brightest possible liposomes and study ways of conjugating antibodies to these liposomes to yield paucivalent particles. The behavior of the immunoliposomes on the cell surface will be characterized in terms of patching, capping and lateral diffusion. Development of the labels should allow application of particle tracking and laser tweezers techniques to a wide range of cell types, included rounded and thick cells whose images are not easily used in present single particle techniques.