This paper reports on the biological and cultural determinants of total, LDL, and HDL cholesterol, and triglyceride (TC, LDL-C, HDL-C, TG) levels using a general linear model on randomly selected family data collected during 1975-1978 at nine North American Lipid Research Clinics. Initially, the analyses were clinic-specific to assess the importance of genetic and cultural transmission, marital resemblance, and other determinants of these traits and then were made jointly to identify the nature and sources of any heterogeneity between clinics. There was evidence of significant genetic and cultural factors for all traits in most clinics. Clinic heterogeneity was also significant, but excluding one clinic reduced the heterogeneity considerably. The genetic (h2) and cultural (c2) heritabilities for the remaining eight clinics were homogeneous with pooled estimates of h2 of .556 +/- .028, .539 +/- .028, .485 +/- .029, and .358 +/- .028, and of c2 of .029 +/- .006, .033 +/- .006, .075 +/- .008, and .089 +/- .009 for TC, LDL-C, HDL-C, and TG, respectively. Among the traits, HDL-C exhibited the most difference among clinics, and both HDL-C and TG showed the largest cultural heritability. The relevance of these and similar studies in a broader understanding of the determinants of plasma lipids and lipoproteins is discussed.