Plasma apo B and apo A-I were determined in 477 subjects (206 males and 271 females) by laser immuno-nephelometry. Measurements of VLDL- + LDL-cholesterol, HDL-cholesterol and triglycerides were done simultaneously. VLDL- + LDL-cholesterol and apo B values were similar in males and females and increased with age. HDL-cholesterol and apo A-I values were higher in females but stable with age. Different regression curves (HDL-cholesterol vs apo A-I) were obtained in males and females and a negative correlation was found between HDL-cholesterol or apo A-I and triglycerides. Increased body weight was associated with higher values of VLDL- + LDL-cholesterol, apo B and triglycerides in both sexes but lower values of HDL-cholesterol and apo A-I essentially in males. Finally, the study provides evidence of a relationship between smoking and alcohol consumption on the one hand, and HDL-cholesterol and apo A-I on the other.