Changes in antigen expression during monocyte to macrophage maturation. The expression of maturation-associated antigens MAX.1 and MAX.3 increases during culture in human serum (•), but not when monocytes are cultured under serum free conditions (▿). Each point represents the mean of five experiments performed in duplicate. Relative OD is the optical density of MAX.1 and MAX.3 divided by the optical density of the HLA class I MoAb (values of isotype specific controls were subtracted before). Statistically significant differences (P < .05) are indicated by an asterisk.