Mutation research has given an insight into the rather complex genetics of kernel rows in barley. At least 12 gene loci can act to promote the spike development, fertility, and kernel development. Mutants with such effects as six-row and intermedium phenotypes show clear morphological distinctions among different loci, but also among different allelic mutants. These genes, without exception, are capable of unexpected synergistic reinforcing or disturbing intraction, the extremes being typically six-rowed or deformed spikes, respectively. The investigations have centered on 69 intermedium mutants, representing 9 loci, in double mutant combinations, in double combinations with the six-row gene hex-v, in triple combinations with hex-v, and in triple and quadruple mutant combinations. The effects of the interaction may differ among the three characters of lateral floret development, among intermedium loci, and among alleles of the particular locus. Particular types of gene interaction are indicated, particular loci being more competent than others, and the particular alleles being more competent in relation to the constellation of loci. Accumulation of intermedium genes in more complex gene systems leads to progressive promotion of lateral floret development, but there are indications that such systems may be more sensitive to environmental stress, leading to irregular or even deformed spike formation. Probably, representatives of the hex-v locus should form the fundamental constituent in the synthesis of gene systems with the most efficient promotion of lateral floret development in the breeding of six-row barley.