Viable cells of Micrococcus luteus secrete a proteineous growth factor (Rpf) which promotes the resuscitation of dormant, nongrowing cells to yield normal, colony-forming bacteria. When washed M. luteus cells were used as an inoculum, there was a pronounced influence of Rpf on the true lag phase and cell growth on lactate minimal medium. In the absence of Rpf, there was no increase in colony-forming units for up to 10 days. When the inoculum contained less than 10(5) cells ml-1, macroscopically observable M. luteus growth was not obtained in succinate minimal medium unless Rpf was added. Incubation of M. luteus in the stationary phase for 100 h resulted in a failure of the cells to grow in lactate minimal medium from inocula of small size although the viability of these cells was close to 100% as estimated using agar plates made from lactate minimal medium or rich medium. The underestimation of viable cells by the most-probable-number (MPN) method in comparison with colony-forming units was equivalent to the requirement that at least 10(5) cells grown on succinate medium, 10(3) cells from old stationary phase, or approximately 10-500 washed cells are required per millilitre of inoculum for growth to lead to visible turbidity. The addition of Rpf in the MPN dilutions led to an increase of the viable cell numbers estimated to approximately the same levels as those determined by colony-forming units. Thus, a basic principle of microbiology - "one cell-one culture" - may not be applicable in some circumstances in which the metabolic activity of "starter" cells is not sufficient to produce enough autocrine growth factor to support cell multiplication.