Very different concentrations of plasma-lactoferrin in healthy adults have been reported in the literature. We compared three commercially available lactoferrins and lactoferrin purified in our laboratory as calibrators in an ELISA. No statistical differences among these preparations of lactoferrin were detected. The concentration of purified lactoferrin was measured by dry weight, and efforts were made in order to minimize loss of purified lactoferrin by adhesion to tubes etc. and thus, secure accuracy of the method. Dilutions were made in PBS 0.01 mol l-1 with NaCl 0.436 mol l-1, (NH4)2SO4 0.5 mol l-1, BSA 5 gl-1 and normal rabbit IgG 10 mg l-1, which was shown to give parallel dilution curves of primary calibrator, secondary calibrator and plasma samples. No significant difference in the content of lactoferrin in neutrophils (median; range) among men (1.78; 0.83-4.48 micrograms 10(-6) neutrophils; n = 20) and women (2.12; 1.16-9.30 micrograms 10(-6) neutrophils; n = 14) was found. Lactoferrin was analysed in EDTA-plasma obtained from 135 female and 227 male blood donors. Median concentrations were 84.7 and 97.8 micrograms l-1 respectively, while 2.5% and 97.5% reference limits (with 90% confidence intervals) were estimated to 42.9 (38.7-47.4) micrograms l-1 and 166.9 (151.0-186.3) micrograms l-1 for women and 52.3 (49.1-55.6) micrograms l-1 and 189.9 (175.9-206.4) micrograms l-1 for men, respectively.