Insulin receptors are present on fetal and newborn tissues in significantly greater numbers than on adult tissues. Recent studies have suggested that membrane fluidity, which is dependent upon lipid constituents, is important in regulating the appearance and behavior of insulin receptors. We have compared the lipid composition and fluidity as well as insulin receptor binding to monocytes from normal adults and full term normal infants. Newborn infants had significantly higher insulin levels than did fasting adults (17.4 +/- 2.4 versus 9.8 +/- 0.6 microU/ml; P less than 0.001); despite this, cord blood monocytes showed significantly higher 125I-insulin tracer binding than did those of adults (9.5 +/- 0.51 versus 7.6 +/- 0.45%/10(7) cells; P less than 0.02). From Scatchard analysis, it was evident that cord monocytes had greater numbers of both high (2.94 versus 1.25 X 10(-10) M-1) and low affinity (13.1 versus 8.57 X 10(-10) M-1) receptors than adult monocytes. Cord mononuclear cells had significantly lower phospholipid concentrations than adult cells (0.085 +/- 0.012 versus 129 +/- 0.012 mg/mg of protein; P less than 0.025) and significant elevations of cholesterol/phospholipid ratios (0.520 +/- 0.045 versus 0.354 +/- 0.009; P less than 0.005). Microviscosity determinations were performed using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Cord cells had significantly greater microviscosity values (fluorescence polarization) (0.339 +/- 0.030 versus 0.186 +/- 0.019; P less than 0.005), compared to adult cells. For all subjects, a highly significant correlation was noted between cell microviscosity measurements (fluorescence polarization) and 125I-insulin tracer binding to mononuclear cells (r = 0.72, n = 15, P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)