Post-gamma globulin previously isolated and partially sequenced in this laboratory was used for production of polyclonal and monoclonal (hybridoma) antibodies. A radioimmunoassay method was developed for quantitation of post-gamma globulin with either antibody. The titration curves obtained were treated statistically and found practically indistinguishable. The sensitivity of the method adopted for the quantitation of post-gamma globulin in a variety of biological fluids was 0.13 ng/ml and the upper limit of precision was 2.5 ng/ml. The following results were obtained (mean +/- 1 SD): normal sera, 0.96 +/- 0.20 microgram/ml; pregnancy sera, 1.08 +/- 0.28 micrograms/ml; cord blood 2.08 +/- 0.33 micrograms/ml; hospitalized patient's sera, 1.3 +/- 0.54 micrograms/ml; geriatric subjects' sera 2.26 +/- 1.10 micrograms/ml; cerebrospinal fluid, 5.37 +/- 3.36 micrograms/ml; saliva, 1.22 +/- 0.67 micrograms/ml; synovial fluid, 1.27 +/- 0.41 micrograms/ml and urine, 0.11 +/- 0.125 microgram/ml. To shed light on the catabolism of post-gamma globulin the levels of beta 2-microglobulin were also measured radiometrically. Correlative statistical analysis of all the data have shown that renal handling of post-gamma globulin and beta 2-microglobulin may be very similar but not necessarily identical.