Abstract Heat-sensitive ht mutants of T2H which failed to complement ambers in genes 6 and 10 in T2H appeared to complement ambers in these same genes in T4D. The results of experiments solving this paradox are described. The heat-sensitive mutant of gene 6 in T2H required either the product of the T2H allele or the T4 ts allele of gene 29 to give a thermosensitive baseplate. The heat-sensitive mutant of gene 10 in T2H required the products of both gene 6 and gene 29 to give a thermosensitive baseplate. Thus, the heat sensitivity of the phage depended on particular combinations of the products of different allelic forms of different genes. Interactions of baseplate proteins are discussed.