Circulating antibodies against oxLDL are present in several inflammatory and autoimmune conditions. Such antibodies are also present in patients with atherosclerosis, although the pathogenic significance of the antibodies is still not known. We have characterized the antibodies with regard to isotype, subclass, affinity and effect on macrophage uptake of oxLDL. Antibodies of IgG and IgM isotype were most common and found both in patients with atherosclerosis and in normal individuals. The subclass of IgG antibodies was mainly IgG2 and IgG3. Scatchard analyses of IgG and IgM antibodies showed that IgG antibodies were heterogeneous with regard to affinity, whereas only one population of high-affinity antibodies was found in the IgM antibody population. The high-affinity populations had an average equilibrium constant (K0) of 8.84 x 10(9) M-1 for IgG antibodies and of 1.65 x 10(9) M-1 for IgM antibodies. Incubation of 125I-oxLDL with purified IgG and IgM from sera with high amounts of antibodies enhanced the uptake of 125I-oxLDL in the monocyte-like U937 cell line. Antibody preparations from sera containing no anti-oxLDL antibodies and from sera with antibodies against LDL had less effect on this uptake. The increased uptake was competitively decreased by adding unlabelled oxLDL. This study shows that antibodies against oxLDL are mainly IgG2, IgG3 and IgM. Both IgG and IgM antibodies have a high affinity for the antigen and increase the uptake of oxLDL in a monocyte-like cell line.