We have assessed the value of p210 protein detection in peripheral blood cells for follow-up of chronic myelogenous leukaemia (CML) patients. Quantification was achieved by comparing the relative intensities of the p210 bands with those of the normal abl protein (p145). Serial dilution of Ph-positive K562 cells with Ph-negative HL60 or KG1 cells revealed a linear correlation between the p210/p145 ratio and the number of Ph-positive cells (r = 0.998; p < 0.001) with a sensitivity of detecting less than 1% Ph-positive cells in 5 x 10(6) cells. Ninety-six consecutive patients were enrolled in the study and a total of 155 Western blot analyses have been performed and compared to chromosomal analyses of bone marrow. Parallel RT-PCR analyses have been performed on 99 occasions. All patients with positive cytogenetic findings also probed positive for p210. In 23 instances p210 was detectable despite negative chromosomal analysis. In 16 samples these results were confirmed by RT-PCR. In patients with partial cytogenetic remission (n = 26) the results of the p210 assay correlated significantly with the percentage of Ph-positive metaphases (r = 0.69; p < 0.001). In conclusion, monitoring of CML patients by quantification of the bcr-abl protein is a feasible and sensitive alternative to chromosomal analysis of bone marrow.