Exponentially growing FSA 1233 cells, 1 of the clones isolated from a mouse fibrosarcoma, were synchronized by fractionation according to cell size by centrifugal elutriation. Cells from each fraction were analyzed by flow microfluorometry to determine the stages of the cell cycle and were injected i.v. to determine lung colony-forming efficiency. In vitro plating efficiency of these cells was similar throughout the cell cycle except for a slight reduction at G2 + M. Lung colony-forming efficiency showed marked cell cycle and cell size dependencies, being lowest at G1, highest at S, and declining slightly at G2.