The cell-surface proteins of Drosophila embryos at gastrula and myoblast fusion stages were characterized by radioiodination and two-dimensional gel electrophoresis. Over 13% of the cell surface proteins detected in gastrula embryos were not found in myoblast fusion stage embryos or in Drosophila embryonic cell line EH34A3 cells. Nearly 18% of the cell-surface proteins detected in myoblast fusion stage embryos were evident only at that stage. Embryonic cell-surface proteins were compared with cell-surface proteins from untreated EH34A3 cells and EH34A3 cells treated with 20-hydroxyecdysone, which induces cell aggregation and the expression of "new" proteins at the cell surface (D. F. Woods and C. A. Poodry, 1983, Dev. Biol. 96, 23-31). Only one of the proteins induced by ecdysone in EH34A3 cells was detected in the NP-40 soluble fraction of radioiodinated cell lysates, even after fractionation by lectin affinity chromatography and immunoprecipitation to enrich for putative ecdysone induced proteins. However, extraction of the NP-40 insoluble pellet of embryo cells revealed one additional protein that was present both in myoblast fusion stage embryos and hormone-treated culture cells. It was concluded that except for these two proteins, the cell-surface proteins induced in cultured cell lines by treatment with 20-hydroxyecdysone are not present in significant amounts in gastrula or myoblast fusion stage embryos.