Modest numbers of blood monocytes become attached at least temporarily to the endothelium of large arteries in normal swine fed low fat, low cholesterol diets. These numbers are increased several fold when the swine are fed a high saturated fat, high cholesterol atherogenic diet (BT). The main objective of this portion of a broader study was to see if the addition of fish oil (30 ml) to a BT diet (BT + FO) could prevent the increase in attached monocytes induced over arterial endothelium in BT fed swine. Six BT, 6 BT + FO and 5 control mash (MA) swine fed the respective diets for 4 months before killing were available for the current study. Other aspects of this experiment have been presented previously which in brief are that BT + FO resulted in retardation of atherosclerotic lesion development and a shift in lipoprotein components from predominantly apolipoprotein B,E containing with the BT diet to predominantly apo B only with BT + FO. There was a significant positive correlation between lesion development and apo B,E lipoproteins. In the current study we determined by scanning electron microscopy on the first portion of the left anterior descending coronary artery after perfusion fixation under pressure the number of monocytes per mm2 attached over or not over visible lesions. We also determined monocyte percentages in the circulating blood and analyzed the correlation of the numbers of attached monocytes and blood monocyte percentages with various lipoprotein components reported previously.(ABSTRACT TRUNCATED AT 250 WORDS)