The distribution of the BLA, CALLA (CD lo), AC-2 (CD 39), MHM-6 (CD 23), LB-I, and 35165 (CD 45R) antigens in 40 non-Hodgkin's lymphomas was demonstrated by immunohistochemical staining of frozen tissue sections. Nine out of 10 centroblastic and centrocytic follicular and diffuse ty e of lymphomas (CWCC F/D) and all 10 cases of CB/CC follEular lymphomas were BLA and CALLA+. A few cases also showed weak expression of activation antigens (AC-2, M H M d and LB-I) and 351C5. In contrast, 3 CC and 3 lymphoblastic (non-Burkitt) lymphomas showed a heterogeneous pattern of distribution with dominating activation antigen expression. A single case of lymphoblastic lymphoma of Burkitt-like type expressed BLA and CALLA but not activation antigens. In reactive follicular center and FCC lymphomas different cell populations appeared to express BLA and activation antigens, respectively. Assessment of staining intensity and proportion of the stained cells indicated that almost all BLA+ cells are CALLA'. CALLA+ BLAcells were regularly present, in addition. The co-expression of BLA and CALLA in the same cell was confirmed by double immuno-enzymatic staining. By the same technique, BLA+ and CALLA+ cells were shown to be activation-antigen negative.