This experiment was conducted to investigate the effects of extracellular CLA on proliferation and differentiation of cultured adipocytes and its dietary supplementation on backfat and intramuscular fat deposition in finishing pigs. Seven-day-old Duroc x Landrace x Large White pigs were killed to obtain adipocytes for culture. Adding 3 forms of CLA (cis-9, trans-11 CLA, trans-10, cis-12 CLA, or CLA mixture) at concentrations of 50, 100, 150, 200, 250, 300, 350, and 400 microM to culture medium for 10 d increased cell differentiation (P < 0.05). In addition, 3 forms of CLA enhanced cell proliferation (P < 0.05) at 50 to 350 microM and inhibited cell proliferation (P < 0.05) at a concentration of 400 microM. Seventy-two Duroc x Landrace x Large White crossbred gilts weighing 60.7 +/- 2.8 kg were randomly assigned to 1 of the 3 dietary treatments (control, 1.25% CLA, and 2.5% CLA). The pigs were slaughtered at 100.0 +/- 5.7 kg. Dietary CLA increased lean percentage (3.5 to 4.7%; P = 0.07), intramuscular fat content (P < 0.05), and shear force (P < 0.05), but reduced 10th- and first-rib backfat depth (P <or= 0.05) and lipid oxidation (P < 0.05). The adipocyte diameter in backfat or LM was not affected by CLA. Supplemented CLA reduced Delta(9)-desaturase activity (P < 0.05) in backfat and LM, as well as fatty acid synthetase activity and lipoprotein lipase activity in backfat, but enhanced (P < 0.05) adipocyte fatty acid binding protein mRNA content in LM. These data indicate that dietary CLA regulates fat deposition by affecting adipocyte proliferation, adipocyte differentiation, gene expression, and key metabolic enzymes of lipid metabolism.