Secretion proteins from larval salivary glands of Drosophila melanogaster were analyzed with acrylamide gel electrophoresis. Four fractions were found; three showed electrophoretic variants in different wild-type stocks. Crossbreeding and cytogenetic techniques were used to localize the genes responsible for the two main fractions: The gene for fraction 3 was found to lie within a segment of the third chromosome which includes section 68C; the gene for fraction 4, Sgs-4, was found to lie within section 3C8-3D1 of the X chromosome (1 - 3-5). The puffs within these sections of the giant chromosomes are active before and during secretion synthesis and become inactive as secretion synthesis ceases. Larvae of one wild-type stock which lack protein fraction 4 do not exhibit any puffing in 3C. The relative amount of protein 4 in the salivary secretion shows a marked dependence on the dosage of the Sgs-4 gene in both duplication and deficiency genotypes. The active site within puff 3C11-12 apparently contains the structural gene for protein 4.