The serum 25-hyroxyvitamin D (25-OHD) concentrations of healthy young hospital laboratory workers in Britain and Denmark were compared in relation to assay variation and vitamin D intake. Serum samples from subjects in London and Copenhagen were assayed in both countries. The Danish assay gave higher values than the British assay with a correlation r = 0.88 and a linear regression of British values on Danish values of y = 0.60x + 3.35. Factors which might account for this difference were differences in the extraction procedure and in the range of values covered by the standards used in the two countries. However, the mean serum 25-OHD concentrations of the Danish group were significantly higher than those of the British group when all the samples were assayed in both countries. The mean total daily vitamin D uptake of the Danish group was significantly higher than that of the British group but this difference was almost entirely due to the regular intake of vitamin D tablets by sixteen of the twenty-five Danish subjects. Only one British subject took supplements. The vitamin D content of the food eaten was similar in the two groups. No correlation was found between dietary, as opposed to supplementary, vitamin D intake and serum 25-OHD levels presumably because the exposure of these subjects to ultra-violet light was adequate to compensate for minor differences in intake.