Based on the observation that H. influenzae can be cultivated in "symbiosis" with A. metalcaligenes in complex media free of hemin (similar to anaerobes in the presence of air), mixed cultures of several physiologically different types of organisms were studied. In filtrates of pure cultures of A. metalcaligenes, but not in filtrates of Escherlchla coli, Bacillus cereus and Staphylococcus aureus, a hemophilus-X factor was found. This factor was of no importance in mixed cultures of A. metalcaligenes and anaerobes; for growth initiation of anaerobes, the catalase function of the anaerobic partner was limiting, as shown by quantitative determination of growth after addition of the inhibitor 3-amino-1,2,4-triazol to the culture medium. These findings are discussed, especially those concerning biosynthesis of prophyrins by heterotrophic microorganisms.