BT474 and SK-BR-3 mammary adenocarcinoma cells contain eight copies of the c-erbB2 gene but overexpress the mRNA 80 times over the levels measured in normal breast or in the HBL-100 cell line. Using Northern blot analysis and molecular titration based on RNAase protection assay, the decrease in the c-erbB2 mRNA level was monitored in BT474 cells treated with actinomycin D from 1 up to 24 h. The c-erbB2 degradation rate during the first 12 h corresponds to a calculated c-erbB2 mRNA half-life of approximately 7 h. Forty percent of the mRNA present in the cells before treatment remains undegraded after transcription has been blocked for 24 h. Pretreatment with cycloheximide results in complete mRNA degradation in 24 h, suggesting that labile proteins stabilize part of the c-erbB2 mRNA population. Comparison with the c-erbB2 mRNA turnover in HBL-100 'normal' cells indicated that the accumulation of the c-erbB2 gene product in the tumor cells is not the result of stabilization of the messenger. Rather, it is correlated with an increased rate of c-erbB2 mRNA transcription as indicated by run-on transcription assays. Both BT474 and SK-BR-3 tumor cell lines were found to synthesize 20-40 times more c-erbB2 mRNA than HBL-100 cells.