In order to survey the distribution along the bacteriophage lambda chromosome of Rec-mediated recombination events, crosses are performed using conditions which block essentially all DNA synthesis. One parent is density-labeled and carries a genetic marker in the left terminal lambda gene (A), while the other parent is unlabeled and carries a genetic marker in the right terminal lambda gene (R). Both parents are deleted for the lambda recombination genes int and red, together with other recombination-associated genes, by virtue of either (1) a pure deletion or (2) a bio insertion-deletion. The distribution in a cesium density gradient of the resulting A+R+ recombinant phage reflects the chromosomal distribution of the recombination events which gave rise to those phage. Crosses employing either of two different pure deletion phage strains exhibit recombinational hot spot activity located near the right end of the lambda chromosome, between the cI and R genes. This hot spot activity persists when unlimited DNA synthesis is allowed. Crosses employing bio1-substituted phage strains exhibit recombinational hot spot activity located to the right of the middle of the chromosome and to the left of the cI gene. Crosses employing either bio1 or bio69-substituted phage strains indicate that the bio-associated hot spot activity occurs in the presence of DNA synthesis, but is dependent on a functional host recB gene.