Dense populations containing 129 x 10(6) Jensen sarcoma, 134 x 10(6) DON Chinese hamster, 28.9 x 10(6) WI-38 human diploid, 61.8 x 10(6) HEp-2 human carcinoma, and 67.4 x 10(6) WISH human amnion cells were produced from dilute inocula, 0.85 to 5.33 x 10(6), in 7 to 8 days in a perfusion system using replicate T-60 flasks. Perfusion rates as high as 560 ml medium/day/T-60 were required to maintain pH (to ca +/-0.1 unit) and adequate nutrient supplies. The cell densities encountered are described by the term "monolayer equivalents" (M.E.), defined as number of cells per culture divided by number of cells in a monolayer. The M.E.'s for T-60 cultures containing unusually dense populations of 40 x 10(6) WI-38 and 250 x 10(6) DON cells (9-day perfusion) were 5 and 17, respectively, and numbers of cells in illustrations of stained cross-sections of membranes from these cultures were in excellent agreement. Threshold M.E.'s exist below which proliferation is the chief cellular activity and above which one or more cell functions may predominate even though proliferation persists. Cellular nutrition and metabolism may change with changes in M.E., as illustrated in different patterns of glutamic acid, proline, and glycine utilization or production in dense vs. dilute WI-38 cell populations. The results indicated that the role of contact inhibition phenomena in arresting cellular proliferation was diminished in perfusion system environments.