Nine retired (on an average of 4 yr) stainless steel welders had higher (P < 0.001) urinary Cr levels [mean 7 (range 3-13) .mu.mol/mol creatinine] than 21 nonexposed referents [mean .ltoreq. 1.5 (range < 0.6-7) .mu.mol/mol creatinine] but did not differ in this respect from 14 active welders studied at the end of a 31-day vacation (mean 9, range 4-17). This result showed the existence of a slow compartment for Cr in the body. Urinary Cr on time after the end of exposure was analyzed mathematically by an exponential 2 compartment model. Good fits were obtained, showing the existence of a fast compartment in addition to the slow one. For 4 welders followed for 31 days, the biological half-time of the slow compartment ranged from 14 days 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 significant (P < 0.01) correlation between Cr in air (total and soluble hexavalent) and urinary Cr (rS = 0.68 and 0.64). The variation of urinary Cr on Cr in air was considerable, especially at Cr air levels at or below the hygienic standards. Correction for urinary Cr levels on Monday morning did not decrease the variation.