A high proportion of Micrococcus luteus cells in cultures starved for 3-6 months in spent medium following growth to stationary phase in batch culture lost the ability to grow and form colonies on agar plates, but could be resuscitated from dormancy by incubation in liquid medium containing supernatant taken from the late log phase of viable cultures of the same organism (Kaprelyants et al. 1994). In the present work, we found that during the first 50-70 h of such resuscitation the dormant cells actually divide for 10-17 generations in lactate minimal medium containing yeast extract whilst remaining nonculturable on agar plates. Further incubation results in a decrease in the total cell number in liquid medium. The addition of viable (culturable) Micrococcus luteus cells in concentrations of up to 10(4) ml-1 to test tubes containing either resuscitating cells or supernatant from these cultures revealed the excretion of a factor or factors which inhibited the proliferation of otherwise viable cells. The maximum production of this factor took place after some 96 h of incubation of starved cells in resuscitation medium. Supernatant from late logarithmic phase batch cultures of M. luteus abolished the antibacterial effect of starved cultures incubated in resuscitation medium. It is concluded that the stimulating effect of viable cells, and of supernatant taken from batch cultures, on the resuscitation of dormant cells might be connected in part with overcoming the activity of an antibacterial factor causing self-poisoning of dormant cells during their resuscitation.