Mouse and rat liver, kidney and mammary gland explants were bombarded with high-velocity microprojectiles carrying a chloramphenicolacetyl transferase gene under different promoters (pTAT-cat, p chi-Casein-cat, p beta-Casein-cat). The expression of a CAT gene was revealed in all organ explants 24 h after transfection. The most pronounced expression was found when a TAT-CAT construction was used. In experiments in vivo rat liver was bombarded in situ with microprojectiles carrying pTAT-cat DNA. A marked activity of the CAT gene was detected 24 h after the bombardment.