Using crossed starch gel immunoelectrophoresis with rabbit anti-Zn-alpha 2-glycoprotein (Zn-alpha 2-GP) immune serum and planimetric evaluation of antigen-antibody precipitation peak area, human serum Zn-alpha 2-GP was found to consist of four major antigenically identical components (A, B, C, D), the mean concentrations of which were 8.3 +/- 3.5 mg/l (A), 27.1 +/- 3.0 mg/l (B), 38.7 +/- 3.4 mg/l (C) and 11.1 +/- 3.5 mg/l (D). The mean relative percentages of individual Zn-alpha 2-GP components were 9.7 +/- 4.0% (A), 31.8 +/- 3.4% (B), 45.4 +/- 5.2% (C) and 13.0 +/- 4.0% (D). The concentrations of human serum Zn-alpha 2-GP components A, B and C increase from the lowest values in the fetal and early newborn period to the highest ones in children and adults. During ontogenesis, the mean relative percentages of the Zn-alpha 2-GP components A and B increase while those of the Zn-alpha 2-GP components C and D decrease simultaneously. The anodal mobility of the Zn-alpha 2-GP components decreases after treatment with neuraminidase. The pattern of the Zn-alpha 2-GP components in human serum was found to be very similar to that in human urine and amniotic fluid.