This study involved 27 geriatric patients with asymptomatic chronic bacteriuria; all had indwelling Foley catheters. The treatment regimens (daily oral dosage) were: methenamine mandelate (MM) granules, 4 gm; MM, 4 gm, plus ascorbic acid, 4 gm; and MM, 4 gm, plus ascorbic acid, 4 gm, plus cranberry cocktail, 1 liter--administered according to a cross-over design. Proteus vulgaris, Pseudomonas aeruginosa and E. coli were the most common urinary organisms. Proteus organisms were more often found in alkaline than in acidic urines, but the type of pathogen had no influence on urinary pH. Urinary formaldehyde concentration [HCHO] was lower in patients with Proteus infection (17.7 micrograms/ml) than in those with Pseudomonas (21.9 micrograms/ml) or E. coli infection (21.8 micrograms/ml). However, for Proteus infection, [HCHO] was higher in patients receiving MM plus ascorbic acid than in those receiving MM alone. Addition of cranberry cocktail to ascorbic acid did not enhance urinary pH, [HCHO] or methenamine efficacy. Our data suggest that in Foley catheter patients with chronic asymptomatic bacteriuria secondary to Proteus, Pseudomonas or E. coli infection, the type of urinary pathogen or the urinary pH cannot be used to predict the efficacy of methenamine therapy either with or without urinary acidifying agents.