This paper presents a detailed analysis of the content and format of seven machine-readable bibliographic data bases: Chemical Abstracts Service Condensates, Chemical and Biological Activities, and Polymer Science and Technology, Biosciences Information Service's BA Previews including Biological Abstracts and BioResearch Index, Institute for Science Information Source Tape, and Engineering Index COMPENDEX. Selected issue test tapes of each data base were printed and checked for the types of data that were contained in the issue and the methods in which the data were formatted. This paper compares the physical formats of the tapes and describes the varied treatments given to such data elements as authors, titles, abstracts, etc. Great discrepancies in the presentation of essentially similar bibliographic data were found, and some suggestions for mitigating the discrepancies by use of standards are offered. (Author)