Following the oral application of 100 mg 59Fe-equivalents in different ferrous iron preparations the whole body retention (R) and erythrocyte incorporation (EI) of absorbed 59Fe as well as the postabsorptive serum increase SII after 3 h were estimated by intraindividual and simultaneous comparison in healthy subjects with normal and depleted iron stores. The regression equation R = (0.0 +/- 0.4) + (1.16 +/- 0.04) x EI and a Spearman rank correlation coefficient r = 0.93 were found to describe the correlation between whole body-59Fe-retention R (mg Fe) and erythrocyte-59Fe-incorporation EI (mg Fe) after 2 weeks. The correlation between the postabsorptive serum iron increase SII (mumol/l) after 3 h and the whole body-59Fe-retention R after 2 weeks is characterized by the regression equation R = (0.0 +/- 0.8) + (0.36 +/- 0.03) x SII and a correlation coefficient r = 0.88. Whole body counting of absorbed 59Fe represents the most sensitive, reliable and only quantitative method for measuring iron absorption or bioavailability in man. If however this method is not available postabsorptive serum increase and it's conversion into iron absorption using the described regression equation is a useful substitute for the at least semiquantitative estimation of iron bioavailability from oral iron preparations.