Cells of Tetrahymena may produce autocrine signal molecules with effects on survival and proliferation. Here we have tested the effects of human recombinant and bovine insulin, and the B22-B30 fragment of bovine insulin over a wide range of concentrations (10(-5)-10(-18) M) on cell survival and proliferation in a synthetic nutrient medium. The cells were grown in conical flasks at low initial cell densities (40 and 400 cells/ml). Insulin prevented rapid cell death and/or promoted cell proliferation over two separate concentration ranges: down to nanomolar levels and again in the low pico- and femtomolar range. At an initial population density of 400 cells/ml the cells multiplied at both concentration intervals. At 40 or fewer organisms/ml the cells multiplied in the high concentration interval, whereas in the low interval they survived for about four times longer than those in the control cultures. B22-B30 added to cultures of 40 initial cells/ml produced a stimulation of cell survival in the low pico- and high femtomolar range. In the presence of hemin (50 nM) cells at 400 initial organisms/ml multiplied at insulin concentrations down to about 3 nM and again from 300 am to 10 pM. In some cases, hemin plus insulin activated cell proliferation between the two concentration intervals as well. At 40 cells/ml the cells not only survived but proliferated in the femtomolar range. Cells in cultures supplemented with both hemin and B22-B30 multiplied at the low concentration interval (from about 100 fM to 10 pM).