The study of dendritic cells (DC) has been hampered by the difficulty of isolating rare cells for analysis of their phenotype and function. Interpretation of the DC lineage has been largely influenced by studies on cell populations which can be readily isolated and amplified in the presence of cytokines. Long term cultures (LTC) from murine spleen have been shown to support continuous in vitro hematopoiesis of DC dependent on interaction with a stromal cell monolayer. LTC-DC represent a single, stable class of DC derived by constant turnover of spleen DC progenitors maintained within stroma. They represent a resident DC population in spleen. The functional characteristics of LTC-DC have been studied in terms of capacity to stimulate T cells and response to activation by environmental stimuli. LTC-DC have many morphological, phenotypic and functional properties reflecting an immature or partially mature, marginal zone-like CD4(-)CD8(-) splenic DC subset. They are highly endocytic and can process and present protein antigen to naive hen egg lysozyme (HEL)-specific MHC-II-restricted TCR-Tg CD4(+) T cells. They do not, however, induce T cell proliferation in a mixed lymphocyte reaction. LTC-DC do not respond in a typical fashion to common DC activators like LPS and CD40L. They upregulate MHC-I and CD80/CD86 but not MHC-II and CD40. They reflect an endogenous, immature DC subset in spleen with properties distinct from immature DC located in peripheral tissues.