AbstractThe two propagative phases of bacteriophage lambda, lysogeny and lysis, can be used in concert to enhance productivity of recombinant expression systems. Lambda vectors carrying mutations to prevent both cell lysis and λ DNA packaging in the lytic state have been shown to yield 100% stability of the product gene in lysogeny and to produce up to 15% of total cell protein as product β‐galactosidase in a mutant lytic state.14 Despite these mutations, partial lysis of the culture was observed following induction of the cells from a lysogenic phase into the lytic state. To understand better the phage‐host cell interactions and to investigate the possible cause(s) of lysis in these highly productive expression systems, we have made a detailed study of the suppressor‐free system JM105(NM1070). We have found high levels of product (15% of total cell protein as β‐glactosidase) to be due chiefly to a high‐copy number of λ DNA in the mutant lytic state. There is partial lysis of the culture even in this suppressor‐free system caused by a low‐level natural suppression of the amber mutation in gene S of NM1070, resulting in accumulation of λ endolysin. We have also monitored changes in cell growth and morphology upon induction of the lysogen. There is a slight increase in cell number that follows a linear relationship with time and a 25‐fold increase in cell volume during recombinat protein production in the mutant lytic state.