Multicell spheroids provide an in vitro tumour model of intermediate complexity between tumours and standard cell cultures. Spheroids of several varieties of normal and tumour cells have been grown and Chinese hamster V79-171b cells have been studied in detail. Spheroids simulate conditions in those solid tumours which contain nodular areas with necrosis at a distance from blood vessels and show a decrease in growth fraction as they increase in size. They contain radiation resistant chronically hypoxic cells which develop spontaneously as the spheroids grow, and exist in a tumour-like environment, yet retain their growth potential. Several assays have been developed or adapted for use in spheroid experiments to study cell properties and responses to treatment. These include many of the techniques used in tumour biology to determine the net response of the tumour, such as volume changes, histology, and net cell survival as well as more direct assays involving separation of subpopulations of cells. Methods were also developed to study in situ repair and repopulation processes after ionizing radiation or other cytotoxic agents.