Previously we have described (Hugo, P. et al., Int. Immunol. 1990. 2: 209) an immature CD4+CD8-CD3- thymocyte subset which is thought to be the counterpart of the CD4-CD8+CD3- subset. In this study we show that the ontogeny of these two subsets is parallel in fetal thymic organ culture. Extensive phenotypic characterization of CD4+CD8-CD3- cells reveals that they closely resemble CD4-CD8+CD3- thymocytes being: HSAhigh, Thy-1high, interleukin 2 receptor alpha chain negative, CD44-, H-2K+/-, CD5low, MEL-14low/intermediate, CD2+, LFA-1+ and MTS 35+. Finally, we show that the proportion of CD4+CD8-CD3- thymocytes is highly variable between mouse strains.