Peripheral blood mononuclear cells of 21 patients undergoing allogeneic bone marrow transplantation (BMT) were monitored post-BMT for immunologic markers (E rosettes OKT3, OKT8, DR, and BT5/9, a monoclonal antibody which stains helper T cells), cytochemical markers (acid phosphatase [AP], beta-glucuronidase [BGLU], and acid alpha naphthyl acetate esterase [ANAE] ), and morphology. The cytochemical T score, was obtained from typical AP, BGLU, and ANAE reactivity. The same was done for the cytochemical non-T score and macrophage (Mo) score. All patients received cyclosporine A (CyA) for graft-v-host disease (GvHD) prophylaxis. In univariate analysis there was no significant correlation between the proportion of E rosettes, OKT3-, OKT8-, DR-, and BT5/9-positive cells, and GvHD. The first three showed instead a positive correlation with time from transplant: E rosettes (P = .02), OKT3 (P = .01), and OKT8 (P = .003). In contrast, a significant negative correlation was found in univariate analysis, between the cytochemical T score and GvHD (P = .0001), and a positive correlation between non-T score and GvHD (P = .0008), as well as between the Mo score and GvHD (P = .03). There was no influence of time from transplant on the T (P = .8), non-T (P = .8), or Mo score (P = .4). In multivariate analysis comparing E rosettes, OKT3, T score, non-T score, GvHD, and time from BMT, the only variable associated with GvHD was the T score (P less than .05). These results suggest that T cell activation during GvHD is associated with a loss of hydrolase expression in T cells, but does not imply relevant modifications of immunologic surface markers. In addition, lysosomal enzymes appear early (before day 10) after transplantation, indicating that T cells at this stage are well differentiated.