Escherichia coli K-12 minicells were employed to examine polypeptides encoded by plasmids carrying wild-type and mutant Tn1 or Tn3 transposition elements. Tn1- and Tn3-containing minicells express high levels of four transposon-specified polypeptides. Three, of molecular weights 30,000, 28,000, and 25,000, are related immunologically to beta-lactamase, the enzyme responsible for ampicillin hydrolysis. A fourth polypeptide of molecular weight 19,000 is encoded by the Tn1 or Tn3 region which spans the BamHI cleavage site. Mutant transposons which no longer produce this polypeptide transpose at higher than wild-type frequencies to give aberrant transposition products (Gill et al., J. Bacteriol. 136: 742--756, 1978; Heffron et al., Proc. Natl. Acad. Sci U.S.A. 72:3632--3627, 1975). No expression could be detected from a region of the transposons extending from the inverted repeat sequence distal to the beta-lactamase gene to more than half the distance into the Tn1 or Tn3 sequence.