In the present investigation we have studied the expression of platelet-derived endothelial cell growth factor/thymidine phosphorylase (PD-ECGF/TP) in ten different human lung carcinoma cell lines, four small cell carcinomas and six non-small cell carcinomas. None of the small-cell lung carcinoma cell lines demonstrated expression of PD-ECGF/TP mRNA. However, four of six of the non-small cell carcinoma cell lines expressed the 1.8 kb PD-ECGF/TP transcript. The cell lines derived from the single squamous cell carcinoma and the two adenocarcinomas expressed the PD-ECGF/TP mRNA, and were found to have the corresponding protein both in cell lysates and conditioned media as determined both by immunoblotting and measurement of thymidine phosphorylase activity. Only one of three studied large cell carcinoma cell lines expressed low levels of PD-ECGF/TP mRNA, but the corresponding PD-ECGF/TP protein was not demonstrated by immunoblotting.