The use of a cyanide ion-specific electrode in combination with the Conway microdiffusion method was modified for the measurement of cyanide concentration in human red blood cells and plasma. With our modified method, the optimalpH of cyanide isolation from red blood cells and plasma was investigated. Cyanide recovery from red blood cells increased with decreasingpH. The maximal recovery of 96.9 ± 2.6% was obtained at apof less than 1. Cyanide recovery from plasma, however, peaked at apH between 7 and 8, and further changes inpH reduced the recovery rate. The maximal recovery rate from plasma was 74.1 ± 1.5%. In previous studies, cyanide isolations from both plasma and red blood cells were carried out at apH of less than 1. This study shows that cyanide isolation from plasma should be performed at apH between 7 and 8.