The purpose of this study was to find how closely a near infrared instrument (IRI), Futrex 5000, would compare to whole body counting (WBC) of potassium-40 (40K) in assessing body composition.We measured lean body mass (LBM) and percent total body fat (PCF) in 177 males and 27 females by total body potassium (TBK) and by infrared interactance (IRI). TBK was assessed by 2 pi whole body counter (WBC) and IRI by Futrex (F) instrument. These subjects were measured for TBK and for IRI within 1 week of each other.A large mean difference (p