The effects of citrate and dialysed urine (dU) on the aggregation of hydroxyapatite (HAP) crystals in solutions with different pH and otherwise with an ion composition assumed to correspond to that in the distal part of the distal tubule were studied by spectrophotometric assessment of the rate of crystal sedimentation. When the concentration of dU was increased from 1% to 20% we recorded an increased inhibition of HAP crystal aggregation at pH 6.5. There were no differences in the inhibition accomplished by 10% dU when the pH was varied between 5.5 and 7.0, but a lower inhibition was recorded at pH 7.5. Citrate in the range 0.05-4 mmol/l had a concentration-dependent inhibitory effect on HAP crystal aggregation. In the presence of 10% dU the net inhibitory effect of citrate was reduced at all pH levels. In the pH interval 5.5-7.0 a higher inhibition was recorded with 0.5 mmol/l citrate than with 10% dU, but in the presence of dU there was only a minor additional effect of citrate at concentrations below 0.5 mmol/l. These findings indicate that urinary macromolecules present in dU strongly inhibit HAP crystal aggregation under solution conditions corresponding to those in distal tubular urine. At the same nephron level citrate might have a direct inhibitory influence on the aggregation of HAP crystals, but in the presence of normal urinary macromolecules the additive inhibitory effect of citrate is probably only marginal.