Growth behavior in vitro of ascites tumor cells was examined for a model study on the invasion in cancer. For this purpose three-dimensional agar culture was used. Seventeen strains of tumor cells were used. These tumor cells grow floating in ascites as single isolated cells (single cell strain) or forming different per cent of cell aggregates (island type strain). Plating efficiencies of tumor cells were variable, depending on the kinds of tumor stain. It was noted that FM3A cells of the cultured cell line showed almost 100% plating efficiency irrespective of the number of cells inoculated. Tumor cells formed 3 types of colony; that is solid type, mushroom-shaped and disk-shaped colony. These three types of colony were observed in all the tumor strains tested, depending on the position in agar layer. The cells of single cell strains showed the tendency to form loose colonies in which cell contact was loose and cells were easy to liberate from the periphery of the colony. The cells of island type strains showed the tendency to form packed colonies in which cell contact was tight. Growth behavior of cells of ascites tumor in agar medium showed good correlation with that in susceptible animals.