T4 mutants defective in any one of the genes 19, 48 and 54 accumulate baseplates which appear complete in electron micrographs. Extracts prepared from 19-, 48- and 54-defective mutant-infected cells complement in pair-wise mixtures to produce infectious phage, indicating that interaction of the baseplate with gene products P19, P48 and P54 to form the core can take place in vitro. Baseplates isolated from each of the three defective extracts by zone sedimentation were tested for ability to complement the other two unfractionated extracts. The results demonstrate that the sequence of gene-product interaction is 48, 54, 19. © 1971.