Cultures of the minimal deviation rat hepatoma cell line H4-11-E derived from the Reuber hepatoma were grown on bundles of artificial capillaries. Sections of those cells grown in circumfusion culture on acrylic copolymer and polysulfone capillaries were prepared. Light, transmission and scanning electron micrographs disclosed variations in cell viability in different parts of the bundle, variations in capillary wall structure affecting the proximity of cells to capillary lumen, and intraluminal sediment deposits derived from pump tubes used for circulating the culture medium. These sediments, by obstructing capillaries, inhibited cell growth and caused focal necrosis of cells. The importance of monitoring the morphology of cell cultures grown on bundles of artificial capillaries which are to be used as artificial organs is emphasized.