CAMPATH-1H is a humanised monoclonal antibody against the CD52 antigen which is being developed for treatment of chronic lymphocytic leukaemia (CLL), autoimmune disease and prevention of transplant rejection. Measurement of antibody serum levels is important for optimising dose regimens but difficult owing to the low concentration compared with normal human IgG. After consideration of various methods, a suitable assay was developed based on indirect immunofluorescence. Test samples were incubated with target cells (HUT-78, a human T cell line) and the CAMPATH-1H was detected by binding of a fluorescent-labelled anti-human Ig using a flow cytometer. Robustness of the assay was demonstrated under a range of experimental conditions. Because of the low affinity of CAMPATH-1H, only a weak signal was seen at low concentrations. The limit of detection was 0.15 microg/ml and the limit of quantitation was 0.25 microg/ml. Since serum samples were diluted at least 1:2, the lowest concentration which can be measured in patient serum was 0.5 microg/ml. The overall precision (coefficient of variation) was +/-13% and the overall accuracy (bias) was +9%. There was a low incidence of false-positive results (<2%) in normal or pre-treatment patient serum. Quantitative recovery was obtained from serum samples spiked with CAMPATH-1H and stored under a variety of conditions, including being treated at 56 degrees C for 30 min and frozen and thawed up to four times. This validated assay is suitable for the measurement of CAMPATH-1H levels in clinical trials and the same principles may be applied to any other cell-binding monoclonal antibody.