A constitutive isotype of serum amyloid A (SAA4) is present mostly in high density lipoprotein (HDL) and a little in other lipoproteins. In this study, we developed an automated method for measuring SAA4 concentration in serum by kinetic nephelometry of anti-SAA4 antibody-coated latex agglutination. Rabbit antibodies generated by immunization of recombinant SAA4 were found to have no apparent reactivity with acute phase SAA. The values determined by this method and by the previous enzyme immunoassay showed good agreement (r = 0.862). Serum SAA4 values of 26 healthy adults ranged from 37-109 mg/L (mean; 62 mg/L). Their SAA4 concentrations were not significantly related with those of apolipoprotein A-I, A-II, B, C-II, C-III or E. Also, SAA4 did not correlate with cholesterol in preparation after removal of very low density lipoprotein and low density lipoprotein. These suggest the unique behavior of SAA4 in lipoprotein metabolism, while what contributes to variation of SAA4 levels in serum, especially in HDL, remains to be clarified.