We have carried out a series of reversible chemical cross-linking experiments using the reagent ethylene glycol-bis(succinimidylsuccinate) with the goal of determining the three-dimensional structure of the bacteriophage T4 baseplate. In a previous report, we investigated the near-neighbor contacts in baseplate precursors and substructures (N.R.M. Watts and D.H. Coombs, J. Virol. 63:2427-2436, 1989). Here we report completion of the analysis by examining finished baseplates and tails. Most of the previous contacts were confirmed, and we report several new contacts, including those within the central hub (gp5-gptd2, gp26-gptd), between the hub and the outer wedges (gp6-gp27(2], between baseplate and sheath (gp54-gp18), and between sheath and core (gp19-gp18). On the basis of this and other available information, a partial three-dimensional model of the baseplate is proposed.