This study demonstrated for the first time that bone marrow is a target of enhanced in vivo monocytopoiesis in hyperlipemia. A significantly greater number of bone marrow cells (BMC) were recovered per femur in swine fed a hyperlipemic (HL) diet compared with swine fed a normal (N) diet. In addition, a significantly elevated number of monocytic precursors proliferated in HL-swine compared with N-swine BMC cultures grown in standardized media in the absence of an exogenous source of colony stimulating factor (CSF). HL-swine sera stimulated a significant enhancement in the number of proliferating monocytic precursor cells, regardless of whether or not the BMC were from HL-or N-swine. In addition, HL-swine compared with N-swine BMC demonstrated an enhanced intrinsic capacity to form monocytic colonies in culture, irrespective of the source of swine sera used to stimulate growth.