A soluble form of CD23 (sCD23) was found in the urine from 12 normal individuals but was not present in 20 normal sera, suggesting that sCD23 produced by cells in tissues is eliminated in the urine. The sCD23 from urine differed in physicochemical properties from the sCD23 found in supernates from B-lymphoblastoid cell lines (B-LCL) and in the sera of patients with B type chronic lymphocytic leukaemia (B-CLL). On SDS-PAGE analysis under reducing conditions urinary sCD23 showed two bands corresponding to molecular weights of 45-60 kD and 28-35 kD indicating that sCD23 may be excreted in combination with another molecule. When subjected to gel filtration in its native state, sCD23 from urine showed a major peak at approximately 150 kD and a minor peak (probably a breakdown product) at 21 kD. Urinary sCD23 was more strongly held by DEAE-cellulose and required 0.5 M buffer pH 8.0 for elution, suggesting that it is more anionic than sCD23 from culture supernates. Five MoAbs recognizing different epitopes on sCD23 from B-LCL supernates were tested on urinary sCD23. Four of the MoAbs were reactive but one (EBVCS-1) was not. Urinary sCD23 did not bind to IgE. The level of sCD23 found in normal urine (approximately 0.02-0.05 micrograms/ml) was exceeded in 17 of 24 cases of B-CLL. In one case with a high cell count and a serum concentration of 10 micrograms/ml, the urine contained 80 micrograms/ml sCD23. In another case a high serum sCD23 was not matched by a high urinary level. In this case the gel filtration pattern was closer to that found with urine sCD23 rather than the B-LCL pattern found with sera of other B-CLL patients.