The polycation protamine sulfate was compared to polybrene, the usual agent employed, for its ability to increase the efficiency of retroviral infection. The murine retroviral vector SAX, which contains the neoR gene and the human adenosine deaminase (ADA) cDNA, was used as a marker of cell infection. SAX viral supernate was titered on NIH 3T3 cells in varying concentrations of polycation. The highest infection efficiency for protamine was seen at 5 micrograms/ml and was 7-fold greater than infections performed in the absence of polycation. Infection efficiency using protamine averaged 92% +/- 11 (SEM) of the highest efficiency obtained with polybrene. Total ADA activity attained when human-ADA deficient T cells were exposed to SAX supernate in the presence of protamine was 83% of that attained with polybrene. The infection rate of mouse bone marrow early progenitor cells (CFU-S) was similar with each polycation. In summary, for supernate infections, concentrations of 5-10 micrograms/ml of protamine provided essentially the same infection efficiency as polybrene with low toxicity on a range of cell types. Since protamine is approved for human use by the U.S. Food and Drug Administration it provides an effective alternative to polybrene when developing human gene therapy protocols.