Asha Soosapilla1, Daniel Quirk 1 , Carmela Brizzi1, , Haiping Sun1 , Ian Bilmon 1, Jane Freeman1 , Kalpesh Golani1 , Basel Alshaer1Kim Vu1 , Lilean Khamo1, Nenna VanBilsen1, Nina Dhondy1, ,Paul Downe1 , Ping Han1, Stephen P. Mulligan1 Background: Immunophenotyping is an essential and well established technique for the diagnosis, classification and monitoring of B-cell leukaemia and lymphoma often by demonstrating monoclonality or a diagnostic phenotype. By contrast, identification of clonal T-cell proliferation using flow cytometry is more challenging due to the lack of specific markers for clonality which previously required T-cell receptor gene re-arrangement studies. Aim: To identify T-cell clones using IOTest® Beta Mark Kit on patients showing aberrant T-cell phenotypic expression. . Methods: V Beta repertoire testing was performed when an aberrant expression of one or more T-cell markers was detected. Beta Mark Kit consists of 8 bottles each containing monoclonal antibodies (mAb) with only two fluorophores for the detection of 3 Vβ expressions. Analysis was performed on the Navios 10 colour instrument. Results: Total number of cases for each Vb restriction were: Vb1–3, Vb2–9, Vb3–6, Vb4–1, Vb5.1–2, Vb5.2–3, Vb7.2–1, Vb8–1, Vb9–3, Vb11–1, Vb12 – 2, Vb13.1–9, Vb13.2–4, Vb13.6–1, Vb14–2, Vb17–8, Vb22–1. Discussion and conclusion: In total, 57 consecutive cases with aberrant expression of one or more T-cell markers in routine flow cytometry testing also revealed restricted usage in one out of 24 possible v β segments. Vb 2 and Vb 13.1 were the most common clonal abnormality noted followed by Vb17. T-cell receptor gene re-arrangement studies were done on 18 cases and monoclonal bands were confirmed on most cases. Therefore, IOTest® Beta Mark kit is a quick and easy technique to detect the clonal T-cell proliferation on patients and can be used to monitor disease progression before and after therapy.