Virus infected cells can carry HLA antigens not demonstrated in uninfected cells. In a wider context, it is known that tumour cells in the mouse can exhibit H2-like antigens of foreign haplotypes which they resemble. Our aim was to investigate, using an absorption method, the HLA antigens of ten leukaemic cells and to compare these results with those of normal lymphocytes. The following sera was used for absorption: anti-B5 serum, anti-B7 serum, anti-B12 serum. These sera were absorbed on the ten leukaemic cells and fifteen healthy lymphocytes. An anti-A1 + B8 serum and an anti-A2 serum were absorbed on the ten leukaemic cells, but there was not enough serum to carry out the similar absorption with the healty lymphocytes. Two points emerged from these results. (1) Leukaemia cells absorbed anti-HLA antibodies as effectively as healthy lymphocytes when the cells carried the antigen corresponding to the serum specificity. The curves obtained with leukaemia cells were comparable to those of healthy cells. (2) When the cells were not carriers of the antigens corresponding to the antibody's specificity, leukaemia cells were also capable of absorbing antibodies, unlike healthy lymphocytes which had no or poor absorption.