Flow cytometry offers significant advantages when compared with techniques such as traditional microfluorometry or autoradiography. First, flow cytometry allows for rapid analysis of distribution within single cells or cell populations. In this way, large numbers of cells from a given population can be analyzed, and even the most rare subpopulations detected. The advent of fluorochromes with increasing specificities and reporter capabilities allows for the simultaneous measurement of multiple parameters within a population of cells, or among distinct cell populations. Advances in staining techniques and conditions which better preserve cell viability and membrane integrity have enabled detailed analyses of cell cycle and antigen expression by flow cytometry. Complementation of flow cytometry with cell sorting provides the additional advantage of physical separation of cells for further study once they have been identified by fluorescence to have unusual or desirable characteristics. This proposal is based on the urgent need to capitalize on these advantages of flow cytometry to achieve specific aims of ongoing NIH-funded research. Investigators will use flow cytometry techniques to achieve these goals, which include: elucidation of structure/function relationships among antimicrobial peptides; characterization of cells which are susceptible or resistant to antimicrobial agents; screening nucleic acid expression libraries; defining the interactions among pathogens, endothelial cells, and neutrophils; and identification of cells which respond specifically to immunomodulatory stimuli by producing cytokines or expressing leukocyte adhesion molecules. Acquisition of the proposed instrument will significantly advance the progress and expand the scope of these studies. Thus, we have identified three principal goals in establishing this research flow cytometry facility on the Harbor-UCLA Medical Center campus: i) to provide an accessible and economical dedicated research flow cytometer for multiple investigators on this campus; ii) to coordinate a flexible network that will enable investigators to perform flow cytometric analyses using fluorochromes and conditions tailored to their specific applications; and iii) to promote technical expertise for investigators with less experience in using modern flow cytometric techniques. These goals will enable investigators to achieve specific aims of ongoing NIH-funded projects, and to generate preliminary data essential for submission of competitive grant applications to the NIH and other agencies. Dedicated space has been allocated by Harbor-UCLA Research and Education Institute to house the instrument and the institution has committed strong support to the development of this essential research facility through coordination of recharge fees, and provisions for maintenance support and overhead costs for its long-term use.