Volunteer adult twins (aged 18-65), after a 12-hour fast, provide two 15 ml samples of blood for analysis by Auto Analyzer II, by agarose electrophoresis and by direct gas chromatography. Provisional diagnosis of zygosity is based on subjects' own opinions, supported by dermatoglyphic analysis and, in any difficult case, substantiated by an enzymatic series. Analysis of the AA-II and electrophoresis data on the first 2 × 45 subjects shows a significant positive correlation within MZ pairs for each of the 7 lipid measurements considered, while the estimate of correlation within DZ pairs is lower in every case. Heritability appears to be higher for cholesterol than for triglyceride fractions. The electrophoretic band for the sinking pre-beta lipoprotein fractions has behaved like a simple genetic marker. Interrelationships among different lipoprotein fractions are studied in terms both of phenotypic and genetic correlations, both of which confirm the expected positive association between amounts of triglyceride and of cholesterol in the supernatant (Sf > 20), and both also show negative correlation between the pre-beta and the alpha fractions. Another aspect of the analysis is the study of lipid differences between MZ cotwins in relation to corresponding intrapair differences in body weight and medical history; male, but not female pairs, show a measurable statistical dependence of cholesterol and triglyceride levels on « excess » body weight.