The leukemic promyelocytes in 37 cases of acute promyelocytic leukemia (APML; FAB, M3) were examined for their cytochemical property. Thirty-two cases (86%) showed strong myeloperoxidase (MPO), chloroacetate esterase (Es-chl) and Sudan black B (SBB) positivity, suggesting a pure neutrophilic differentiation of the leukemic cells. However, in 5 out of 37 cases, a strong, diffuse alpha-naphthyl acetate esterase (Es-a) positivity, which was sensitive to sodium fluoride treatment was observed in addition to strong MPO, Es-chl and SBB positivity. This suggested monocytic differentiation of a proportion of APML cases. In 31 cases, surface marker studies were carried out with the help of a panel of monoclonal antibodies consisting of two panmyeloid antibodies (GM 58/8 and 1G10), one anti-HLA-DR antibody (7.2) and one 'myeloid' antibody (5F1) with restricted reactivity with monocytes (CD14). The purpose of including the monoclonal antibodies 5F1 and 7.2 was to determine if a correlation could be established between strong Es-a positivity and reactivity of the leukemic promyelocytes with 5F1 and 7.2 in individual cases. All the 5 cases with 'monocytoid' cytochemistry were unreactive with 5F1, and only one case in this group showed 15% 7.2-positive cells. The lack of immunophenotypic support for the monocytic cytochemistry of the 5 cases of APML suggests that the monocytic phenotype of leukemic promyelocytes is both aberrant and incomplete. Since normal promyelocytes are purely neutrophilic, this could be a manifestation of an 'intralineage infidelity' in APML, similar to that observed in other types of acute leukemia. The clinical significance of 'monocytic' phenotype of APML is not clear; cases with monocytic differentiation did not show different clinical and hematological features when compared to the more common, pure neutrophilic variety.