Thymineless death (TLD) as well as deoxyribosideless death (DRLD) can be observed in Lactobacillus acidophilus R-26 during growth in media lacking thymine or deoxyriboside respectively. Both phenomena exhibit the same interval of lag period (2--3 h) but the rate of inactivation is 2--3 times faster in TLD. Transfer experiments show that inactivation of bacterial reproduction is accelerated immediately if --DR medium is replaced by --T one. In the opposite case the deceleration of the inactivation rate does not appear immediately but after a 1 -- 2 h lag period, in which no changes in the number of viable bacteria can be observed. Our results suggested that the accumulation of deoxyriboside compounds has no causal role in the inactivation of bacterial reproduction. However, the presence of deoxyribosides can accelerate the process of inactivation.Apart from the thymine requirement, another specific precursor of DNA-deoxyribose, is required by this strain (Sow and Lark, 1966). Thymine is essential in the absence of folic acid (Sow 1966). Deoxyribose can be utilized only in the form of deoxyriboside or deoxyriboside-monophosphate. The growth of bacteria in the absence of deoxyribosides results in a loss of reproductive activity similarly as growth in the absence of thymine. The last phenomenon, thymineless death (TLD), has been described by Cohen and Barner (1954). Bacterial death observed during deoxyriboside starvation has been named deoxyribosideless death (DRLD) (Reich and So~ka 1967).TLD and DRLD are similar in many respects as e.g. concerning requirement of RNA and/or protein synthesis. But one basic difference exists: TLD exhibits a 2 to 3 times faster rate of inactivation than DRLD (Reich and Sow 1967). There are two possibilities considered as an explanation of this fact: either the presence of some deoxyriboside compounds in the bacterial pool can accelerate the process of inactivation, or on the contrary, the absence of such compounds can decelerate a process resulting in reproductive inactivation. The results we have obtained suggest that the first alternative is more probable.The characteristic course of reproductive inactivation during transfer of Lactobacillus acidophilus R-26 from the --T to the --DR conditions and vice versa are described in present work. Special attention was paid to the possible role of deoxyriboside compounds as factors determining rate of TLD and DRLD.