Macromolecules resembling soluble immune complexes can be made from heat-aggregated human gamma globulin (AHGG). In 15 rats, we studied vascular trapping of 125I-labeled AHGG (AHGG)-125I) given by constant I.V. infusion over 1 hour while tissue blood flow was marked by intermittant aortic arch injections of 85Sr-labeled microspheres. Red cells labeled with 51Cr (RBC-51Cr) were also infused so that when the tissues were removed at the end of the experiment, the vascular volume of each tissue specimen could be balculated to correct issue 125I for AHGG-125I which was not trapped but simply in transit in the bascular space at the time the tissue was removed. These data permitted us to calculate the fractional uptake of AHGG-125I (FM) for a given tissue in comparison to any other tissue. We chose to compared the FM of each tissue to the FM of renal cortex. This comparison was expressed as a ratio termed the FM ratio for the given tissue. The following tissues had FM ratios significantly greater than 1.00 (i.e., per unit blood flow, these tissues trapped AHGG-125I more avidly than renal cortex): liver, spleen, skin, stomach, fat, testes, and large bowel. The respective ratios were 381 +/- 74, 15.7 +/- 4.0, 11.8 +/- 4.0, 7.47 +/- 1.95, 6.24 +/- 1.0 +/-, 3.03 +/- 0.67, 2.86 +/- 0.72 (all p less than 0.025). The FM ratio for adrenal, heart, thymus, and diaphragm were not significantly different from 1.00. The FM ratio of lung and brain were significantly less than 1.00: 0.014 +/- 0.008 and 0.14 +/- 0.065, respectively (p less than 0.001 for both). In 13 experiments, glomeruli was 23.8 +/- 3.5 per cent as assessed by recovery of the microspheres contained in renal cortex. Compared to whole renal cortex, the isolated glomeruli contained only minor amounts of AHGG-125I. We conclude that tissues vary widely with respect to their ability to trap macromolecules. When uptake is viewed in terms of the amount of complex trapped per unit delivery rate, many organs trap AHGG-125I for more avidly than renal cortex. Furthermore, under the present experimental conditions, glomeruli are not the major intrarenal site of macromolecule uptake.