The relative distribution of T cell subsets as defined by MABs was analyzed in PBMCs from 56 patients with various hematologic diseases. The percentage of PBMCs expressing the CD5 antigen revealed normal values shortly after transplantation. In most patients lymphocytes expressing CD8 were found in high proportion and CD4 in a low percentage. This resulted in a low CD4/CD8 ratio compared with that of lymphocytes from normal controls. In the course of time, a trend toward normalization was observed for all parameters investigated, however the kinetics of the recovery showed a marked heterogeneity. Nevertheless, we found a statistical correlation between the CD4+ cell count and the PHA transformation response. This could however, not be shown in ConA and PWM stimulated proliferation. No other correlation was found between any lymphocyte phenotype, as defined by MABs, and proliferative responses in vitro.