The trans-10, cis-12 isomer of CLA (t10, c12 CLA), synthesized by ruminal microflora, has been reported to be a potent inhibitor of preadipocyte differentiation in rodents. Conversely, arginine promotes the differentiation of 3T3-L1 preadipocytes. We hypothesized that arginine would promote differentiation by up-regulating expression of PPAR, in bovine primary preadipocytes in culture, and that this effect would be antagonized by t10, c12 CLA. The objectives of this study were to establish conditions for the culture of preadipocytes from bovine adipose tissue and to document interaction between arginine and isomers of CLA on differentiation. Bovine stromal-vascular (SV) cells were collagenase-liberated from perirenal adipose tissue of 16-mo-old, corn-fed Angus steers. Extracted RNA from the cells was hybridized to antisense RNA probes. The expression of stearoyl-CoA desaturase (SCD) and fatty acid synthase (FAS) was undetectable in preconfluent preadipocytes, but increased strongly after 7 d of treatment with 10g/mL insulin, 10g/ mL holo-transferrin, and 5M pioglitazone, especially in the absence of dexamethasone. The t10, c12 CLA isomer decreased SCD and FAS gene expression in a dose-dependent fashion when added during the last 3 d of differentiation; the cis-9, trans-11 CLA isomer was without effect except at the highest concentration tested (100 M). Similarly, t10, c12 CLA (but not c9, t11 CLA) depressed the monounsaturated fatty acid: saturated fatty acid ratio in differentiating preadipocytes and depressed acetate incorporation into fatty acids. PPAR, SCD, lipoprotein lipase, TNF and Pref-1, but not C/EBP, exhibited greater expression in preadipocytes treated with 5 mM arginine during differentiation than in control preadipocytes. The t10, c12 CLA isomer (at 40 M) strongly decreased SCD and PPAR expression, even in the presence of 5 mM arginine. We conclude that arginine can up-regulate differentiation of bovine preadipocytes, and t10, c12 CLA strongly antagonizes this effect. Copyright 2006 by New Century Health Publishers, LLC.