The presence of flavin derivatives in baker's yeast was tested by high-performance liquid chromatography and thin-layer chromatography. In yeast samples, besides flavin adenine dinucleotide and flavin mononucleotide, small amounts of riboflavin and traces of 10-formylmethylflavin have been found. Total amount of flavins was calculated to be 17.9 +/- 2.9 micrograms/g of fresh yeast. The distribution of flavin adenine dinucleotide, flavin mononucleotide, riboflavin and 10-formylmethylflavin in total flavin content were estimated to be 71.5%, 25.8%, 1.7% and below 0.05%, respectively. In some samples we have additionally detected small amounts (0.8% of total flavins) of new flavin derivative which has been identified as 4',5'-riboflavin cyclic phosphate by means of its chromatographic and chemical behaviour. This compound seems to be a product of flavin adenine dinucleotide degradation and probably has been earlier mistaken for flavin mononucleotide. Its formation is dependent on pH conditions.