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In an analysis, by both crystallographic and microbiological methods, of 50 urinary calculi recently removed by surgical operation, 33 proved to be of metabolic origin (mostly calcium oxalate and some uric acid or urate) and 17 of ‘infective’ origin (struvite, apatite or a mixture of the two). Metabolic stones were usually bacteriologically sterile or contained only small numbers (< 103/g of stone) of bacteria which did not produce urease, while infective stones always contained urease-producing organisms, usually Proteus mirabilis, in large numbers (> 105/g). The combined approach of stone analysis by crystallography and microbiological culture yields more information than conventional techniques on which to base the treatment of urinary calculi and the prevention of their recurrence.