The protein binding of cyclosporine (CyA) in plasma was studied in serial samples for 6 months after transplantation in 66 renal transplant recipients. Analysis was performed with a recently developed method of equilibrium dialysis in steel chambers. Among the 1,848 samples, the free fraction ranged from 0.5 to 4.2%, with a median of 1.30%. The free fraction of CyA was highest immediately after transplantation (1.66 \Pm 0.49%). Diabetics had a higher free fraction of CyA than nondiabetics during the first 2 weeks after transplantation. A weak, but significant, covariation was observed between the serum concentration of albumin or bilirubin and the free fraction of CyA (r= −0.47 andr= 0.39, respectively;p< 0.01). There was also a significant correlation between serum HDL cholesterol (r= −0.39 to −0.48;p< 0.01), as well as apolipoprotein Al (r= −0.35 to −0.49;p< 0.01), and the free fraction of CyA. A significant drop in the free fraction of CyA was observed immediately prior to acute rejection episodes as compared with I week earlier (p< 0.01), but there was no difference in the free fraction of CyA between patients who lost their graft and those who did not. There was an overall eightfold variation in the free fraction of CyA in plasma, with an up to fivefold intraindividual variation and a 2.3-fold variation in the mean free fraction between individuals. A significant covariation was observed between the free fraction of CyA and the concentration of binding proteins in plasma, as well as factors such as time after transplantation, diagnosis, and clinical events. Thus, it is now possible to monitor free CyA, and this may prove useful in special situations, for example, in patients with dyslipoproteinemias.