The cytochemical reactivities of 3 acid hydrolases, alpha-naphthyl acetate esterase (ANAE), acid phosphatase and beta-glucuronidase were investigated in normal peripheral blood lymphocyte subsets defined by monoclonal antibodies OKT3, 4, 8 and FMC4 (anti-Ia). A combined monoclonal antibody-immunocolloidal gold/cytochemical staining procedure was used to determine enzyme activities and distributions of reaction product in each subset. Cytochemical profiles for each lymphocyte subset were defined. The majority (greater than 85%) of T cells (OKT3+) were positive for all 3 enzymes whereas a minority (less than 40%) of B cells (FMC4+) displayed reactivity. The cytochemical profiles of T helper/inducer (OKT4+) and T suppressor/cytotoxic (OKT8+) cells were not significantly different and corresponded to that observed for OKT3+ cells; thus none of these enzymes can be used to distinguish normal lymphocyte subsets cytochemically. ANAE reactions were further analysed, in the respective subsets, on the basis of dot-like or scattered/diffuse reactivity. The ratios of cells displaying dot-like: scattered/diffuse reactivity, in the respective subsets, were OKT3+, 5.4:1; OKT4+, 8.1:1; OKT8+, 2.4:1; FMC4+, 0.4:1. The cytochemical profiles and ANAE reactivities of T cell subsets identified by monoclonal antibodies differ from those displayed by T cell subsets defined by Fc receptors and confirms that there is little correlation between subsets defined by these two methods.