We have used a combination of subtractive cloning and differential screening techniques to identify genes preferentially expressed in early stages of human T/NK cell development compared with mature T and NK cells. A fetal liver-derived cytoplasmic (c) CD3+ membrane (m) CD3- clone, FL508, which expresses markers characteristic of pre-T and pre-NK cells served as a cell source for our cloning experiments. A cDNA library enriched for genes expressed in FL508 was constructed by removal of cDNA that hybridized to mRNA from a B cell line, JY. One-tenth of the resulting library of 5000 clones was screened by differential hybridization with cDNA probes from JY and a mature CD4+ T cell clone, B21. The relative expression levels of six selected clones were analyzed in 17 different cell/tissue types by semiquantitative polymerase chain reaction. Four of these clones are expressed at higher levels in thymocytes than in mature T or NK cells, and three clones are expressed at higher levels in fetal liver cells than in either mature T or NK cells. Partial and complete DNA sequence information suggests that these six cDNA correspond to previously unidentified genes. Genes identified in this study may be useful not only as markers for early stages of T/NK cell ontogeny, but also as tools for understanding novel developmental events.