Direct microinjection of DNA by glass micropipettes was used to introduce the Herpes simplex virus thymidine kinase gene into cultured mammalian cells. When DNA was delivered directly into the nuclei of LMTK-, a mouse cell line deficient in thymidine kinase activity, 50--100% of the cells expressed TK enzymatic activity. In contrast, no TK activity could be detected when the DNA was injected into the cytoplasm. The number of injected LMTK- cells capable of indefinite growth in a TK+ selective medium (that is, transformants) depended on the nature of the plasmid DNA into which the HSV-TK gene was inserted. One cell in 500-1000 cells which received nuclear injections with pBR322/TK DNA gave rise to a viable colony when grown in HAT medium (that is, a TK+ selective medium). The transformation frequency increased to one in five injected cells when specific SV40 DNA sequences were also introduced into the HSV-TK plasmid. With the microinjection procedure transformation frequency was relatively insensitive to DNA concentration and did not depend on co-injecting with a carrier DNA. Most of the transformants were stable in nonselective medium as soon as they could be tested.