Abstract An abnormal elongation of cells occurred when Lactobacillus bulgaricus B5b was cultivated in milk heated at 100°C for 15min. Nuclear staining revealed that the elongated cells were multinucleate, and septum staining indicated that the septum had not yet formed. Similar cell elongation was confirmed using other strains of L. bulgaricus. Extent of cell elongation varied with strains and heating conditions of milk; severe heating shortened the cell length. This cell elongation was not observed in autoclaved milk (121°C, 15min), milk containing added formic acid and heated to 100°C for 15min, and in mixed culture with Streptococcus thermophilus. The formate added to the milk was incorporated into purines of ribonucleic acid and deoxyribonucleic acid of the cells. Addition of formic acid or adenine stimulated ribonucleic acid synthesis in the cells cultivated in milk heated to 100°C for 15min. The cell elongation was always accompanied by a remarkable depression of ribonucleic acid synthesis. Lactobacillus bulgaricus cells grown in milk heated to 100°C for 15min are not able to produce adequate amounts of formic acid required for synthesis of purines de novo.