We have measured the clonogenic survival of cells isolated directly fr
om colon cancer surgical specimens and treated with 5-fluorouracil (5-
FU). Enzymatically disassociated cells were incorporated into hybrid s
pheroids, consisting predominantly of nonproliferating HeLa feeder cel
ls. Aliquots were exposed for 1.5 hr to a range of concentrations of 5
-FU. From the decrease in clonogenic survival, as deduced from the fre
quency of colony formers among hybrid spheroids after chemical treatme
nt, we were able to construct survival curves in 50% of the surgical s
pecimens tested. A striking revelation was the presence of a resistant
plateau in the survival curves, reminiscent of the solid tumor respon
se to treatment with 5-FU. This resistance was absent in monolayer cul
tures. Evidence is presented that this resistance is due to the absenc
e of, or delay in, cell cycle progression of cells residing in hybrid
spheroids.