1. At the present time there is no method whereby the completeness of 24 h urine collections can be accurately assessed when clinical studies are undertaken. The suitability of 4-aminobenzoic acid (PAB) given with meals as a marker for completeness of urine collections was therefore investigated. 2. When a single dose of 80 mg of PAB was given to four volunteers 93% was recovered in the urine in 5 h. 3. Eight volunteers living in a calorimeter, where complete urine collection could be guaranteed, were given various doses of PAB divided up throughout the day. 88 +/- 5% was excreted in the urine over a 24 h period. Urine excretion and oral dose were directly related. 4. Thirty-three reliable free-living volunteers eating their normal diet took 80 mg of PAB with meals (240 mg/day). Mean urine recovery over the 24 h period was 223 +/- 9 mg, or 93 +/- 4% of the administered dose. The range in individual recovery from maximum to minimum was 15%, compared with 75% for creatinine excretion per kg fat-free mass. 5. PAB is a safe marker of the completeness of 24 h urine collections. Any collection containing less than 205 out of 240 mg (85%) of PAB, given as 80 mg with each of three meals, is probably incomplete.