Incorporation of coconut oil (CO) rich in lauric acid into the milk diet induces a lipid infiltration of the liver (steatosis) in 1-month-old calves. Among possible steps involved in diet-induced liver steatosis, the ability of the calf liver to synthesize apolipoprotein (Apo) B and to secrete it as part of VLDL particles was investigated. Liver samples were taken from calves fed for 17 d on a conventional milk replacer containing CO (n 5) and beef tallow (BT, n 4) as reference. Samples were cut into slices 0.5 mm thick and subsequently incubated for 12 h in a medium containing a [S-35]methionine-[S-35]cysteine mix and 0.8 mm-sodium laurate or oleate, the major fatty acids of CO and BT diets respectively. Concentrations of total [S-35]proteins, [S-35]albumin and [S-35]ApoB in liver cells were 2-fold lower (P=0.08, 0.0004 and 0.03 respectively) in CO- than in BT-fed calves. Although the total amount of proteins secreted (including albumin) was similar in both groups of calves, the amount of VLDL-[S-35]Apo secreted was 2-fold lower (P=0.004) in CO- than in BT-fed calves. These results suggest that a CO-enriched milk diet induces in preruminant calves a lipid infiltration of the liver by decreasing ApoB synthesis, leading to a reduction in secretion of VLDL particles.