Circulating immune complexes (CICs) in sera from patients suffering from chronic myeloid leukemia (CML) at initial diagnosis, in 'remission', at relapse and in blastic crisis have been quantitated using fluid [125I]Clq binding assay in terms of per cent binding activity and microgram/ml aggregated human globulin (AHG) equivalents. The Clq binding activity (Clq-BA) has been compared within the groups of CML patients in different phases of the disease as well as with sera obtained from normal healthy donors. The results showed that the mean Clq-BA was significantly increased in CML patients at initial diagnosis (25.74 +/- 3.48, p less than 0.001), in relapse (53.36 +/- 6.9, p less than 0.001) and in blastic crisis (60.5 +/- 8.7, p less than 0.001) when compared to control sera. Sera of 'remission' patients showed significant decrease in Clq-BA when compared to sera collected in active phases of the disease, however, the values were still significantly higher (12.87 +/- 1.58, p less than 0.02) than those of normal healthy donors. When the levels of CICs as assessed by Clq-BA were compared with the WBC/blast counts of CML patients in chronic as well as blastic phase, it was noted that the variations in numbers of circulating leukemic cells do not correlate with the CIC levels. The significance of assessment of CIC levels in monitoring the disease in CML patients is discussed.