Peripheral blast cells from patients with acute lymphoblastic leukaemia were cultured in the diffusion chamber system. On different harvesting days the cALL antigen, surface immunoglobulin and T-cell antigen were investigated by direct immunofluorescence. Each cell type investigated showed a different response throughout culture, the commonest change observed being development into the B-cell axis. Our data demonstrate that the differentiation stop of a leukaemic cell can be overcome by using the diffusion chamber system. The data therefore give some insight into the stage of developmental arrest in acute lymphoblastic leukaemia.