Seeds of monosodium urate caused heterogeneous nucleation of calcium oxalate at pH 5.7 and 6.7, and of calcium phosphate at pH 5.3, 5.7, and 6.7 from metastably supersaturated solutions in vitro. Seeds of uric acid had a small or no effect. The results could account for the formation of calcium stones among patients with hyperuricosuria and normocalciuria.