Transformation by human adenovirus type 5 requires the cooperation of gene products from both the E1a and E1B early transcription units. Our major goal was to better understand the individual roles that the E1B proteins play in the transformation process. In order to determine the specific contribution made by the two major E1B proteins, 19K and 58K, mutants were constructed which were defective in the synthesis of each protein. Analysis with these mutants suggested that 58K appeared to be necessary for efficient plaque formation on human HeLa cells whereas 19K was not required. Mutants which failed to produce 19K or made a truncated 19K product displayed the cyt/deg phenotype characterized by production of large plaques and degradation of DNA These properties were not apparent with point mutants at methionine 120 or serine 164 of 19K or with mutants defective for 58K production. All E1B mutants produce E1A at levels comparable to wild type adenovirus 5, suggesting that neither E1B protein affects the regulation of E1A expression. Of interest was the observation that in combination with E1A, both 19K and 58K were able to induce transformation of baby rat kidney cells. However, the efficiency of transformation was greatly increased if both these E1B products were present. It seems likely that the mechanism of transformation involving each of these E1B proteins utilizes different pathways, but these pathways appear to be additive. / Thesis / Master of Science (MS)