The relationship between the concentration of ionized magnesium and total magnesium was investigated. Ionized magnesium was determined by an ion-selective electrode (Microlyte 6, KONE) and the result was adjusted to pH 7.4. Total magnesium concentration was measured by flame atomic absorption spectrometry. Total and ionized magnesium were only closely related in marked hypermagnesaemia (> 1.2 mmol/L), but correlation was poor in samples with slightly elevated total concentration or in hypomagnesaemia (< 0.65 mmol/L). The relationship was dependent on protein concentration. The agreement between total and ionized magnesium was acceptable in normoproteinaemia, but in hypoproteinaemia (< 40 g/L) total magnesium concentration was classified in 35% of the samples as below or within the reference interval, whereas the pertinent ionized magnesium concentration was normal or elevated, instead. Studies on paraproteinaemic sera clearly demonstrated that albumin concentration is most important for the size of the protein bound fraction.