Cystotomies were performed on 152 animals [cats, rabbits and dogs] with various suture materials. The bladders were examined for the presence of concretions at intervals of 3-120 days. Calculi were found regularly in rabbits but not in cats or dogs, irrespective of the suture material used. Urinary calculi persisting later than 7 days postoperatively were seen only in rabbit bladders repaired with nonabsorbable sutures. Despite major differences in the geometry and chemical composition of the suture, the 2 absorbable sutures (polyglycolic acid and catgut) showed an equivalent incidence of early, reversible calculus formation in the rabbit urinary bladder. The property of suture materials that contributes most to the formation of persistent urinary calculi was nonabsorbability. Surface characteristics and cross-sectional geometry appear to play little or no role in the calculus formation in the bladders of animals.