Below a critical cell density of 750 cells ml(-1), and in a manner familiar throughout much of cell culture technology, Tetrahymena thermophila die within a few hours despite being supported by a nutritionally complete synthetic medium, SSM, in which a supracritical inoculum (1000 cells ml(-1)) nevertheless proliferates and quickly reaches 1 x 10(6) cells ml(-1). The kinetics of cell death, and the conditions required to keep cells alive at and below the critical density have now been more fully investigated. Interestingly, cell death follows first order kinetics, with a half-life of less than two hours at 250 cells ml(-1). Survival can be extended by an order of magnitude, however, when protein synthesis is reduced by inoculation of cells at this density in: (a)Tris/HCI-buffer;(b) SSM deficient in an essential amino acid (arginine or phenylalanine); or, (c) SSM containing cycloheximide. In the presence of actinomycin D, the critical density required for proliferation can be lowered to 100 cells ml(-1). These results are discussed in relation to the capacity of Tetrahymenaio produce and release signal molecules (loosely referred to as growth factors), which need to be present above a certain threshold level before proliferation occurs. The evidence for the demise of cells at low density being active - in terms of requirement for, or dependency on, new transcriptional and translational processes - is discussed, along with more general implications of the findings for the control of cell death in populations of 'free-living' unicellular organisms in culture compared with their normal habitats.