Blood monocyte‐macrophages from homozygous familial hypercholesterolaemic (FH) patients showed the same pattern of morphological and biochemical changes during maturation in vitro as cells from normal subjects. The protein content of both types of cell increased eightfold during the first two weeks of culture while the ratio of free cholesterol to protein in the cells remained constant. The specific activities of lysosomal enzymes increased between days 2 and 5 and again between days 7 and 12. The specific activity of 5′‐nucleotidase increased rapidly during the first two days and then fell.

Degradation of low‐density lipoprotein (LDL) by cells from FH patients was not saturable and, when expressed per mg cell protein, gradually declined as the cells matured. Normal cells maintained in medium containing whole serum degraded LDL by an additional, saturable process that was, at all times, decreased if the cells were preincubated in medium with a high concentration of LDL and increased after preincubation with lipoprotein‐deficient serum. Saturable degradation of LDL, expressed per mg cell DNA, was highest between the fifth and twelfth days of culture, which was the period of greatest cell growth. The results suggest that normal cells incubated in 20% whole serum expressed LDL receptors to obtain the cholesterol required for their growth, but that the lack of these receptors in cells from FH patients did not prevent normal development.

Saturable degradation of acetylated LDL, or LDL complexed with dextran sulphate, was the same in cells from FH patients as in cells from normal subjects. The degradation of each was lower than that of unmodified LDL for the first 5 days in culture, but rapidly increased between days 5 and 9. This increase corresponded with the development of the phagocytic properties of the cells, as indicated by the release of lysosomal enzymes during incubation with opsonized zymosan.