Two isoforms of the src-family kinase p59 Fyn have been shown to arise through the alternative splicing of exon 7; these have been designated FynT and FynB. Here we describe a novel isoform in which exon 7 is deleted. cDNA's were identified from PBL's isolated from all seven individuals we tested, and semi-quantitative PCR indicates that this delta-7 fyn transcript is expressed at near wild-type fyn levels. We show that the message isolated is functional using an in vitro translation system and by expression in transfected COS cells. We also have evidence suggesting the presence of the delta-7 protein in native cells. Exon 7 contains the ATP binding-site of the kinase and the lack of kinase activity of delta-7 Fyn is confirmed by phosphorylation studies in permeabilized cells. We suspect that delta-7 Fyn may serve an adapter function similar to that shown for mutants of the related Lck lacking a functional kinase domain.