We have developed an improved method to analyse stable associations (conjugate formation) between effector and target cells. Hydroethidine (red) stained lymphoblastoid target cells were cocentrifuged with carboxyfluorescein diacetate acetoxymethylester (green) stained human IL-2 activated cytotoxic cells (LAK). In the present studies either enriched or purified CD3 negative large granular lymphocytes (LGL) were used as cytotoxic cells. These fluorescent vital dyes localize intracellularly and therefore do not modify the cell to cell contact which eventually leads to the lytic events. Both dyes can be excited at a common wavelength (488 nm) using a single argon laser. Effectors firmly bound to target(s) (stable conjugates) were detected as two color fluorescent events (red and green). This method has several features: (a) the number of conjugates is recorded with reference to a fixed number of target cells; (b) the composition of conjugates (number of effectors or targets per conjugate) can be studied by analysis of the fluorescence intensities (red or green); (c) conjugate formation can be studied at E:T ratios comparable to those used in the classical 51Cr release cytotoxic assay; (d) it gives reproducible results and permits the study of very weak differences in binding properties. This method was used to study conjugate formation between human IL-2-activated cytotoxic cells (or purified CD3 negative LGL) and various lymphoblastoid target cells. We were able to demonstrate that cell lines susceptible to lysis formed more conjugates and were surrounded by more LAK effectors than their resistant counterparts and that no conjugate contained more than one target.