were also more typical with the K 12 recipient than with ~~2 0 0 4 -6 in that there were sharp entry times for the markers studied and linear increases of recombinants with time. The possibility that host-controlled restriction decreased genetic recombination between E. coli K 12 and c~2004-6 was pursued. We obtained no evidence for this possibility. Instead the data suggest that genetic heterology between these strains is affecting recombination. Genetic heterology also explains why 0 4~2 behaves more like a K 12 donor in mating experiments with CF 2004-6.

We have discovered different host specificities in the 220 and ~~2 0 0 4 -6 strains of a particular Escherichia coli 0 4 serotype. The host specificities are controlled by a chromosomal locus linked to threonine. They are differentiated on the basis of results from plating efficiencies of phage 004, conjugation, and generalized transduction. The host specificities of E. coli strains K 1 2 and ~~2 0 0 4 -6 seem to be similar and distinct from that of 220.

An auxanographic technique is described, which makes it possible to screen 2n combinations of nutrients on relatively few agar plates.