To investigate the new Yoghurt starter, Lactobacillus helveticus YM-1, which was selected among 14 Lactobacillus strains, and Streptococcus thermophilus CH-1 were inoculated together in reconstituted non-fat skim milk medium and their growth characteristics and cultural conditions for the mixed culture were examined. The main results of this study were obtained as follows. The typical symbiotic growth was shown between the two strains and pH and temperature for optimal growth were 6.5 and 4$0^{circ}C$, respectively. Heat treatment of milk was most effective at 10$0^{circ}C$ for 30 min. The cell-free filtrate of Lactobacillus helveticus YM-1 had stimulatory effect on Streptococcus thermophilus CH-1 but the reverse case was slightly observed. Significant difference was observed in the proteolytic activities between Lactobacillus helveticus YM-1 and Streptococcus thermophilus CH-1. The former liberated 135$mu extrm{g}$ free amino acid per $mell$ of cultured milk, the latter 35$mu extrm{g}$ per $mell$.