1. Unmodified human plasma is not a satisfactory culture medium for human tissues owing to the susceptibility of human fibrin to digestion by tissue ferments. The necessary framework is thus destroyed before the cells begin to migrate. The difficulty can be overcome by adding to human plasma or serum a small quantity of fowl or pigeon plasma, the fibrin of which is highly resistant to digestion. Human tissues have been propagated in this medium for several months through subcultures, and growth in vitro can probably be maintained indefinitely. 2. Human tissues show no greater sensitiveness to changes in temperature and mechanical injury associated with preparation of cultures than those of lower animals. They may be preserved in an ordinary ice box at 10–15°C. as long as 6 or 8 days. Tissues obtained at operation give best results, but pieces of organs removed at autopsy 1 to 4 hours after death sometimes show active growth. 3. The presence of normally existing iso-antibodies (agglutinins and hemolysins) in human serum is without influence on the growth of human tissues in vitro. In other words, autogenous serum has no advantage in tissue cultures over homologous serum.