Abstract In enzymatically produced suspensions of mouse sarcoma cells a DNasesensitive extra cellular gel, probably DNP, appears, entrapping cells. To prevent such clumping adequate amounts of DNase are needed, occasionally several times the concentrations recommended for enzymatic techniques. The amount of DNase required probably varies from tumor to tumor. In addition an instant spontaneous primary aggregation of cells is recorded when tumor cells are suspended in chemically defined media. Aggregation does not appear in balanced salt solutions, probably a devitalization phenomenon. Aggregation can, however, be inhibited and already formed aggregates can be dissociated by the presence of serum or cell-free ascitic fluid. It is recommended that these factors should be checked when strictly single cell suspensions of tumor cells are required.