Nine retired (on an average of four years) stainless steel welders had higher (p less than 0.001) urinary chromium levels [mean 7 (range 3-13) mumol/mol of creatinine] than 21 nonexposed referents [mean less than or equal to 1.5 (range less than 0.6-7) mumol/mol of creatinine] but did not differ in this respect from 14 active welders studied at the end of a 31-d vacation (mean 9, range 4-17). This result shows the existence of a slow compartment for chromium in the body. Urinary chromium on time after the end of exposure was analyzed mathematically by use of an exponential two-compartment model. Good fits were obtained, showing the existence of a fast compartment in addition to the slow one. For four welders followed for 31 d, the biological half-time of the slow compartment ranged from 14 d to infinity. For 12 welders followed for 60 h, the fast compartment had a median half-time of 7 (range 4-35) h. For 19 welders there was a significant (p less than 0.01) correlation between chromium in air (total and soluble hexavalent) and urinary chromium (rs = 0.68 and 0.64). However, the variation of urinary chromium on chromium in air was considerable, especially at chromium air levels at or below the hygienic standards. Correction for urinary chromium levels on Monday morning did not decrease the variation.