The question of nonideality in phage recombination, that is, the extent to which recombinant frequencies differ from those expected from the proportions of the two parental types in the mass culture, was addressed by experiments with lambda tandem duplication phages. Isolation and genotypic analysis of triplication-phage progeny, all of which must be the result of intermolecular recombination, yielded a value of about 0.5 for the nonideality parameter h, i.e., the frequency of unlike-parent matings was only about 1/2 the "ideal" value. This value was independent of multiplicity and about the same for the Rec or Red recombination systems. Similar analysis of single-copy phage progeny yielded estimates of k, the ratio of intramolecular to intermolecular recombination of about 1/6 for the Rec system; no intramolecular events were detected in Red-mediated crosses. Consideration of known nonideality factors (finite input, limited number of intracellular sites for phage growth) suggests that the observed h values correspond to intracellular mixing efficiencies of 55 to 100%, depending on the number of intracellular phage growth sites assumed. Analysis of long-range positive correlation (negative interference) indicates that statistical effects caused unlike-parent double crossovers to be three to four times as frequent as an independent-event calculation would predict. In addition, Rec-mediated crosses showed a 1.3-fold positive correlation for unlike-parent crossovers (in a second interval) among the progeny of like-parent recombinations.