By using phase-contrast microscopy combined with a fluorescent staining technique, the frequency of blast-type atypical lymphocytes (BTALs) appearing in peripheral blood and the phenotypic expression of their surface antigens were studied in 24 patients with schizophrenia, 16 with mood disorder and 14 healthy controls. BTALs were classified as being stimulated or activated cells, morphologically characterized by their large size, dark cytoplasm, a hollow perinuclear containing a few granules and finely dispersed chromatin structures with a few evident nucleoli. A significantly higher number of BTALs were found in the schizophrenic patients compared with healthy control subjects or patients with mood disorder. Further, there was a significant difference in the frequency of BTALs between patients with mood disorder and healthy control subjects. No significant difference in the frequency of BTALs was found between the schizophrenic patients with and without medication. Immunostaining of BTALs revealed that these cells consisted of B, T and non-B, non-T cell subpopulations. Contrary to our expectations, the T cell was only one third of the BTAL population. HLA-DR and CD38 were expressed on most BTALs (> 70%), while CD25, an early activation marker of T cells was rarely found on BTALs (< 0.3%). The differences in activated lymphocyte populations which appeared as morphologically atypical in the circulation among some psychiatric patients and infectious or autoimmune diseases are discussed. This is the first report on populations of BTALs.