The growth-enhancing effects of culture filtrates of respiratory pathogenic bacteria, including Haemophilus influenzae, as well as normal floral bacteria other than Neisseria perflava and Branhamella catarrhalis on L-forms of H. influenzae were examined in vitro, using five species of major respiratory pathogenic bacteria and seven species of normal floral bacteria commonly isolated from the sputum of patients with chronic respiratory tract infections. The growth-enhancing factor(s) was present in the filtrates prepared from the culture of respiratory pathogenic Streptococcus pneumoniae and Staphylococcus aureus, the effects of which were as potent as those of a culture filtrate of B. catarrhalis used as the positive control. The culture filtrates of respiratory pathogenic Pseudomonas aeruginosa and Klebsiella pneumoniae had weak growth-enhancing effects on H. influenzae L-forms. The culture filtrates of 21 strains of normal floral bacteria isolated from the sputum including alpha-hemolytic Streptococci, non-hemolytic Streptococci, Micrococcus roseus, coagulase-negative Staphylococci, and Neisseria spp. had growth-enhancing effects on the L-forms of H. influenzae. These data elucidate the significance of L-forms of H. influenzae in recurrent infections due to H. influenzae in patients with chronic respiratory tract infections.