We describe a method of embedding 100 or more different tissue samples in a normal-sized paraffin block, the multitumor tissue block (MTTB). MTTBs allow the simultaneous immunohistologic testing of numerous tissue samples on a single slide with one drop of antibody. Because all of the tissue samples are treated equally during immunostaining, most sources of variation are removed and comparative studies are facilitated. MTTBs containing miscellaneous neoplasms are suitable for screening of hybridoma supernatants during the early phases of monoclonal antibody production. This permits economical and rapid screening for tissue-specific monoclonal antibodies and simultaneous selection of those that perform well in tissue sections. MTTBs are also useful for the routine monitoring of sensitivity in immunohistochemistry because they can be prepared to include tissues with a broad spectrum of antigen densities. Because a large number of nearly identical sections can be obtained from a single MTTB, these sections could be useful for large scale interlaboratory quality control studies.