Monolayer cell cultures offer distinct advantages when compared to other in vitro systems: Known long-term viability, uniform and extensive exposure of cells to the incubation medium and feasibility for initiating many replicate cultures in any given experiment, allowing the examination of many test conditions. Islet cell cultures are used to examine islet cell replication, islet hormone production, and functional interrelatonships of islet A-, B-, and D-cells. A superfused islet cell culture system will be used in some of these studies. Cultures of human fibroblasts from diabetic and nondiabetic donors will be used to examine hormone action, cell growth, and cell function which may be relevant to pathogenic mechanisms in diabetes mellitus. These studies may distinguish characteristics of diabetes present in cultured cells, determine whether they may be modified, and examine certain metabolic processes such as collagen synthesis which may have relevance to some of the complications associated with human diabetes mellitus.