SUMMARYWe analysed the T cell receptor (TCR) V°repertoire in BAL and peripheral blood (PBL) of three mild stable atopic asthmatics and two non-asthmatic controls. We used the polymerase chain reaction (PCR) to establish the expression of the four V°families, and to detect oligo or monoclonal expansion of°± T cells, we resolved the PCR products on denaturing and non-denaturing gels to find the extent of junctional diversity arising from differences in the lengths of the V(D)J junctions. We also subcloned and sequenced the PCR products to characterize fully the sequence diversity. BAL T lymphocytes from two asthmatic patients (treated with inhaled steroids) expressed only V°II and, in one of them, V°IIJ°u sage was restricted to J°P and J°P1 gene segments, contrasting with the V°J°repertoire found in his respective PBL. Analyses in denaturing and non-denaturing gels showed that the BAL V°IIJ°P and V°IIJ°P1 PCR products resolved into few bands, suggesting deletions at the junctions due to oligoclonal expansion. BAL T lymphocytes from the third asthmatic (not receiving inhaled steroids) expressed V°I, II and III, and the sequences of the in-frame TCR transcripts from this asthmatic and one healthy volunteer who expressed a similar BAL V°TCR repertoire showed clonal expansion of T cells expressing all three V°families. Our analyses showed that much of the°± T cell population found in BAL fluid of humans derives from clonally expanded T cells.