Rat Tamm-Horsfall glycoprotein (THG) was isolated from urine and disaggregated by gel filtration. Purity was confirmed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography. Antibodies were raised and used in the development of a specific radioimmunoassay for rat THG. Parallelism could be demonstrated between the standard preparation and urine (diluted to 1:320 with distilled water) and serum (at all dilutions) samples. Recovery of standard added to urine or serum was on average 93% and inter- and intra-assay variation were both below 7%. There was a positive correlation between the mean urinary excretory rate of THG (μg/24h) and body weight in female rats (p < 0.001) but not in male rats. Serum THG levels were similar in both sexes and showed a positive correlation with body weight (p < 0.001; range 43–240 ng/ml). It is concluded that the rat urinary THG excretion rate is similar to that in man and that rat serum also contains a THG-like protein.