A precursor head of phage lambda is synthesized after induction of cells lysogenic for lambdaD(-)F(-) and lambdaA(-) (head-defective) mutants. This precursor head can be assayed by complementation in vitro and can be purified by CsCl gradient centrifugation and sucrose gradient sedimentation. The precursor head contains no DNA and has the same dimensions as the petit lambda particle. It can be packed with lambda DNA in an extract from induced Escherichia coli lysogenic for a lambdaE(-) mutant.