The complement of the "obligatory" B vitamins presented in part 1 of this review (133:192-199, 1979) refers to factors that must always be supplied to maintain health in a normal individual. The group of compounds that have been designated as "conditional" B vitamins will be discussed in this section and, in addition, a discussion of vitamin C will be presented. The term conditional, implies that members of this diverse group of factors are essential dietary components only under certain special circumstances. Members of this group include niacin, biotin, pantothenic acid, choline, and inositol. When adequate tryptophan is supplied in the diet, humans have the ability to synthesize niacin. Thus, the exogenous requirement for niacin is conditional on the amount of dietary tryptophan. Both biotin and pantothenate are synthesized by intestinal microflora; therefore, a dietary need for either is exceptional. An avian protein, avidin, binds biotin and prevents intestinal absorption. Only