We describe a system for genetic analysis in gene lamb. It consists of a phage which allows mapping, complementation and sequencing studies of lamB mutations and of the sequence of gene lamB. We present results obtained with this system for a set of mutations conferring tight resistant to phage lambda. This leads to a first identification of three residues in the LamB protein which are important for adsorption of phage lambda h+. Residues 151 and 382 are important for reversible adsorption while residues 401 is important for irreversible adsorption. We briefly review other tight mutations affecting gene lamB and discuss some implications of the results on phage receptor interactions, LamB-protein structure and membrane insertion.