We show that antisense RNAs transcribed from genes that are stably integrated into the genome can be used to inhibit the expression of an endogenous cellular gene. Drosophila tissue culture cells were stably transformed with a gene encoding a heat-inducible RNA complementary to the message for hsp26, one of the small heat shock proteins. These cells produced much less hsp26 after heat shock than did untransformed cells. The inhibition was highly specific: expression of the closely related heat shock proteins hsp22, hsp23, and hsp28 was unaffected. By varying the copy number of the antisense gene, the degree of inhibition was varied over a broad range. Reducing the rate of hsp26 synthesis did not appear to affect the synthesis of any other protein during either heat shock or recovery.