The growth factor needs of Haemophilus somnus, which have not been defined to date, were found to be provided by 1% IsoVitaleX (IVX; BBL Microbiology Systems) in tryptose broth. Some growth, however, occurred in unsupplemented tryptose broth. Of the ingredients of IVX, cocarboxylase was found to stimulate growth to about the same degree as the total supplement. Cocarboxylase was without direct effect in 2% peptone broth, which supported no growth of 25 H. somnus strains until supplemented with IVX, optimally at the 10% level. This could be substituted for by proportional amounts of cysteine or cystine, but by no other IVX ingredient. Cysteine-cystine and IVX but not cocarboxylase supplementation allowed H. somnus to grow in Eagle minimal medium, a completely synthetic medium, but attempts at serial passage were unsuccessful.