Normal and malignant plasma cells were investigated for the expression of seven cellular adhesion molecules by immunofluorescence microscopy. The antigens investigated were CD2 and its ligand, LFA-3 (CD58). LFA-1 alpha (CD11a) and LFA-1 beta (CD18) and their ligand ICAM-1 (CD54), H-CAM (lymphocyte homing receptor; CD44) and N-CAM (CD56). Marrow from 18 patients with myeloma, two with plasma cell leukaemia (PCL), four with monoclonal gammopathy of uncertain significance (MGUS) and 10 normal allogeneic bone marrow donors was studied. All plasma cells from normals and multiple myeloma patients were negative for CD2, CD11a and CD18. All normal and myeloma marrow plasma cells were positive for ICAM-1. 16/18 myeloma cases tested, and all other samples (normal, MGUS and PCL), contained plasma cells positive for H-CAM. Only one normal, but 12/16 myelomas tested were positive for N-CAM (P less than 0.02). One of four MGUS cases was moderately positive and one other weakly positive for N-CAM. Both PCLs were N-CAM negative. 12/18 myelomas were positive for LFA-3, but only two normals (P less than 0.05). All MGUS cases were negative for LFA-3, as was one PCL, the other being weakly positive. Three cases were negative for both adhesion molecules, three cases expressed only N-CAM or LFA-3 and 10 cases expressed both. LFA-3 and N-CAM are expressed significantly in myeloma rather than normal plasma cells. Cases of MGUS may express N-CAM but not, in this small series, LFA-3. Plasma cells in the peripheral blood (PCL) and plasma cell lines express little or no LFA-3 or N-CAM.