A simple technique has been used experimentally to produce in vitro Chinese hamster ovary cells with growth fractions ranging from 0 to 100%. Known numbers of exponentially growing and plateau-phase tissue culture cells were mixed in various proportions to yield the desired final growth fraction. Cells attach to the culture flask surface within 1 hr of mixing. Treatment at that time with the nitrosourea compounds, CCNU and MeCCNU, resulted in differential drug survival sensitivities that were dependent upon the growth fraction of the population treated.