In 160 non-Hodgkin lymphomas (NHL) of B and T cell origin the expression of leucocyte function-associated antigen-1 (LFA-1) and one of its counter-structures named 7F7-antigen was studied. Functional and structural similarities and the 7F7-reactivity of pICAM-1 transfected COS-cells prove that 7F7-antigen is identical with ICAM-1. The expression of both adhesion structures occurred variably and clearly depended on the maturation stage and on the B and T cell origin of lymphomas but was not associated with the proliferative status of tumour cells. The concomitant expression of both adhesion molecules was a characteristic feature of germinal centre derived NHL, particularly of those with neoplastic follicular structures (corr. contingency coeff. = 0.506; P less than 0.02), whereas both adhesion structures were less often expressed on lymphomas with a more diffuse pattern of tumour infiltration. Most B-NHL of low-grade malignancy expressed LFA-1 while their counterparts of high-grade malignancy often tended to be LFA-1 negative. In B cell neoplasias of low-grade malignancy the lack of ICAM-1 and a leukaemic course of the disease were significantly correlated (P less than 0.0005). The results indicate that the differential expression of both adhesion molecules may account for distinct patterns of growth and spread in subtypes of lymphoid malignancies.