The relative phage yields of cells of Escherichia coli infected with both wild-type and amber mutant phages deficient in head morphogenesis were determined. The decrease in burst size as a function of the ratio of mutant:wild-type-infecting phage was linear and proportional for mutants in genes 20, 22, and 23, while for mutants in genes 21, 31, and 24 the results suggest an excess of intracellular gene product. The initiation of assembly of phage particles was not delayed at reduced gene product levels; only a reduction in the rate of phage assembly was observed. The effects on burst size of pairs of mutations in genes 20 and 23, 22 and 23, and 22 and 24, in both cis and trans arrangements, were identical. Experiments using the mutant E920g in gene 23 show that varying the kind and intracellular amounts of the major capsid protein (gp23) with respect to the major core or scaffold protein (gp22) had a profound effect on the length of the T4 head. Head length determination must therefore depend on the proper intracellular balance between these two proteins.