The total and ultrafiltrable citrate of serum were measured in healthy controls, and in patients with calcium-containing kidney stones classified into the various calciuria types (normocalciuria, renal, absorptive, resorptive hypercalciuria). The total citrate in two subgroups (normocalciuria, renal hypercalciuria) was significantly higher than in controls. Two independent analyses showed a mean ultrafiltrable fraction of 0.86 (controls and stone formers, with the exception of resorptive hypercalciuria) and 0.95 (resorptive hypercalciuria), i.e., the calculated extent of binding of citrate to serum macromolecules (greater than 10 000 Daltons). The differences in the fraction of free citrate between controls and renal stone formers are not significant. The apparent mean association constants are (1/mol) 0.24 x 10(2) (controls) and 0.29 x 10(2) (normocalciuria). There is a high correlation between the ultrafiltrable fraction and total citrate in the serum, and also between the ratios urinary/serum creatinine and urinary/serum total citrate, during a 2 h endogenous creatinine clearance in the morning (fasting). These findings suggest that 1) there is citrate binding in the serum, 2) the normal portion of free citrate in total serum citrate of normocalciuric stone formers cannot explain the decreased citrate excretion in 24 h urine of these subjects, 3) under defined conditions of examination (morning; fasting) urinary citrate is determined largely by the filtered load of citrate in the proximal renal tubule.