ABSTRACT. In addition to the lipoprotein-deficient d >1.25 fraction, haptoglobin was detected in the high-density lipoprotein (HDL) and the very high-density lipoprotein (VHDL) fractions from sera of calves with experimental pneumonia and cows with naturally occurring fatty liver. It was not found in the chylomicrons, very low-density lipoprotein and low-density lipoprotein fractions. Washing of the HDL fraction did not decrease the haptoglobin concentration. Transferrin and immunoglobulin G were immunoblotted to examine the possibility of contamination of the lipoprotein fractions by the d >1.25 fraction. The two serum proteins were detected only in the d>1.25 fraction, not in any lipoprotein fractions. The distribution pattern of haptoglobin in the lipoprotein fractions was distinct from that of serum albumin. Concentrations of haptoglobin in the HDL fractions from pneumonic sera were largely proportional to those in whole sera. Cholesteryl ester concentrations were decreased in sera from calves with pneumonia, as in cows with fatty liver. A protein immunologically related to hemoglobin was also detected in particular in the VHDL fractions from sera of both groups. These results suggest that haptoglobin or a complex with the hemoglobin-like protein may have a role or roles related to the lipid metabolism.-KEY WORDS: bovine, cholesteryl ester, haptoglobin, hemoglobin, high-density lipoprotein.J. Vet. Med. Sci. 61(2): 119-124, 1999 Holstein calves weighing 72 to 96 kg were used. A suspension of Pasteurella haemolytica (serotype 1, I29 strain; 1 × 10 9 colony forming units) was separately administered to right lungs of 10 calves using a fiber-optic bronchoscope [34]. Ten other calves received the vehicle (phosphate-buffered saline; PBS) alone and were used as controls. Serum was collected at 0 (1 hr before administration) and 0.25 (6 hr after), and at 1, 2, 3, 4 and 7 days after treatment. Calves were exsanguinated at day 1 (3 control and 3 inoculated calves), day 2 (3 control and 3 inoculated), day 4 (2 control and 2 inoculated) and day 7 (2 control and 2 inoculated), to examine pathologic changes and also to collect bronchoalveolar lavage fluids. Preparation of lipoprotein fractions: This was done as described previously [30], with slight modifications. Briefly, 4 ml of serum was overlaid with 2 ml of a solution of d=1.006 and centrifuged at 114,000 × g for 16 hr. The resulting top layer (1 ml) was collected as a mixture of chylomicrons and the very low-density lipoprotein fraction (CM-VLDL; d<1.006), and the next 1-ml layer was discarded. To the bottom layer (4 ml), 2 ml of a solution of d=1.182 was added, mixed and centrifuged at 114,000 × g for 20 hr. The top 1-ml layer was saved as the low-density lipoprotein fraction (LDL; d<1.163) and the next 1-ml layer was again removed. The resulting bottom layer (4 ml) was mixed with 2 ml of a solution of d=1.478 and centrifuged at 114,000 × g for 40 hr to collect the top 1-ml layer as the HDL (d<1.21). After removal of the next 1-ml layer, 0.88 ml of the d=1.478 s...