Lecithincholesterol acyltransferase (LCAT) activity levels were determined, as function of plasma total cholesterol (TC) in 13 normocholesterolemic (TC<85 mg/dL) and in 28 hypercholesterolemic (TC>98 mg/dL) pigs. The normocholesterolemic group consisted of pigs that carried apo-B allelic genes other than Lpb 5 and or Lpb 8. The hypercholesterolemic group consisted of Lpb 5/x and Lpb 5/8 heterozygous and Lpb 5/5 homozygous animals. The data reported in this study show that the LCAT activity in the plasma of hypercholesterolemic (HC) pigs (79±43 units) was significantly lower (p<0.0005) compared to the normocholesterolemic controls (175±45 units). Furthermore, LCAT activity was positively correlated with TC in the normocholesterolemic group (r=+0.54; p<0.05), whereas it was negatively correlated with TC in the hypercholesterolemic group (r=-0.73; p<0.001). Additional data obtained from incubation experiments suggest that the lower LCAT activity in hypercholesterolemic pigs may be due, at least in part, to inhibition of LCAT activity by components found in the lipoprotein-deficient fractions of the plasma of hypercholesterolemic pigs.