Deletion mutants of the Drosophila hsp 70 heat-shock gene have been assayed in COS cells using a vector that contains an SV40 replication origin. COS cells are SV40-transformed monkey cells that support high-level replication of the vector. S1 mapping of transcripts shows that the hsp 70 gene is heat-inducible in these cells, whereas the herpes virus thymidine kinase gene is not. Residues -10 to -66 of the hsp 70 gene are apparently sufficient for heat-inducible promotion, and residues between -47 and -66 are necessary. This region upstream of the TATA box contains sequence features common to other heat-shock genes. Expression of the hsp 70 gene can be forced at low temperature by SV40 sequences that include the 72 bp repeat, but only if these are present on the 5' side of the gene. It seems that the upstream element of the hsp 70 promoter is analogous to that of other promoters, but is only functional in heat-shocked cells.