To understand the targeting mechanisms of peroxisomal serine:pyruvate aminotransferase, in vitro import experiments were carried out using this 43 kDa peroxisomal enzyme, which was synthesized in a coupled transcription/translation system. Being different from other peroxisomal enzymes, such as acyl-CoA oxidase and urate oxidase used in previous in vitro import experiment, the soluble form of peroxisomal serine:pyruvate aminotransferase was fairly resistant to proteinase K digestion. However, the enzyme recovered in the peroxisomal fraction was proteinase K sensitive. This indicates that the association of peroxisomal serine:pyruvate aminotransferase with the peroxisomal membrane causes a marked conformational change in the structure of this protein.