• Hypermagnesemic neonates, born to mothers receiving magnesium (Mg) sulfate before delivery, have elevated ionized calcium (iCa) concentrations in serum. We hypothesized that this elevation might be due in part to competition of Mg with calcium (Ca) bound to proteins or complexed to salts. Serum and its ultrafiltrate were obtained from five placental blood samples. The iCa concentration was measured in both serum and its ultrafiltrate at five incremental Mg concentrations (Δ Mg) up to 6.58 mmol/L. In both serum and ultrafiltrate, incremental iCa (Δ iCa) correlated significantly with Δ Mg. By multiple regression analysis, Δ ica in serum correlated positively with Δ Mg in serum and negatively with albumin concentration in serum. Thus, iCa concentration in serum is increased in vitro by hypermagnesemia; it appears that Mg may compete with Ca bound to albumin and complexed to salts.