The sensitivity of direct solution hybridization of hepatocytes solubilized in guanidium thiocyanate (GuSCN) for detecting α1-acid glycoprotein and albumin mRNAs was studied. The sensitivity of detection was inversely correlated with the DNA concentration. Raising the hybridization temperature from 20 to 37 or 50°C (with formamide) increased the hybridization efficiency three- to fourfold in cell lysates with a high DNA concentration (1 μg/μl), whereas the hybridization efficiency was already maximal at 20°C in diluted samples. It was most important to normalize all hybridization reactions with an internal standard, such as sense mRNA, because of the great variation in hybridization efficiency from one cell preparation to another depending on the DNA concentration. Direct hybridization of GuSCN cell lysates labeledin vivowith [6-¹⁴C]orotic acid was more efficient than hybridizing equivalent amounts of purified [6-¹⁴C]-labeled RNA, perhaps because of greater mRNA integrity and/or better recoveries of mRNA in GuSCN cell lysates. Therefore, direct solution hybridization of GuSCN-solubilized hepatocytes, which avoids the problem of RNA purification, appears to be a rapid, sensitive, and reliable method for quantifying mRNA in hepatocytes.