Confocal laser scanning microscopy and fluorescence microphotolysis (also referred to as fluorescence photobleaching recovery) were employed to study the transport of hydrophilic fluorescent tracers through complement and perforin pores. By optimizing the confocal effect it was possible to determine the exclusion limit of the pores in situ, i.e. without separation of cells and tracer solution. Single-cell flux measurements by fluorescence microphotolysis yielded information on the sample population distribution of flux rates. By these means a direct comparison of complement and perforin pores was made in sheep erythrocyte membranes. In accordance with previous studies employing a variety of different techniques complement pores were found to have a functional radius of approx. 50 A when generated at high complement concentrations. The flux rate distribution indicated that pore size heterogeneity was rather small under these conditions. Perforin pores, generated in sheep erythrocyte membranes at high perforin concentrations, were found to have a functional size very similar to complement pores. Furthermore, the functional size of the perforin pore seemed to be relatively independent of the dynamic properties of the target membrane since in two cell membranes which are very different in this regard, the human erythrocyte membrane and the plasma membrane of erythroleukemic cells, the functional radius of the perforin pore was also close to 50 A. A perforin-specific antibody reduced the functional radius of perforin pores to 45 A.