Objectives: Application of a HPLC (high performance liquid chromatography) method, using cyanide derivatisation, to the determination of plasma pyridoxal-5-phosphate (PLP) concentrations as an indicator of vitamin B 6 adequacy. Setting: The study was performed at the Institute of Food Research, Norwich, UK. Blood samples were taken at the Institute, at Health Centres, or in the volunteer's home. Subjects: 51 adolescent, 131 adult, 68 non-institutionalized elderly and 44 aged ( b 73 y) volunteers were recruited from local authority schools, local Health Centres and General Practitioners. Results: The mean PLP recovery was 92.8%. The intra-and inter-assay coef®cients of variation were 2.8% and 5.2% respectively. Mean PLP concentrations for males and females, respectively, were: adolescents (13 ± 14 y), 36.4 and 43.5 nM; adults (20 ± 64 y), 39.2 and 40.0 nM; elderly (68 ± 73 y), 34.8 and 35.3 nM; aged ( b 73 y), 57.8 and 49.0 nM. Percentages of subjects with PLP concentrations`34.4 nM were over 26% in all population groups. Mean vitamin B 6 intakes (mgag protein intake), as assessed by weighed dietary records, were all above reference nutrient intakes (15 mgag protein). Conclusions: An HPLC method, using cyanide derivitisation, has been applied to the determination of plasma PLP. Comparisons of results for local population groups with current cut-off values for plasma PLP, show large numbers of volunteers at risk of vitamin B 6 de®ciency although this is not re¯ected by vitamin B 6 intakes calculated from food tables. The 34.4 nM cut-off value for value for plasma PLP, indicating de®ciency, is questioned.