We previously isolated two human L1 elements (L1.2 and LRE2) as the progenitors of disease-producing insertions. Here, we show these elements can actively retrotranspose in cultured mammalian cells. When stably expressed from an episome in HeLa cells, both elements retrotransposed into a variety of chromosomal locations at a high frequency. The retrotransposed products resembled endogenous L1 insertions, since they were variably 5' truncated, ended in poly(A) tracts, and were flanked by target-site duplications or short deletions. Point mutations in conserved domains of the L1.2-encoded proteins reduced retrotransposition by 100- to 1000-fold. Remarkably, L1.2 also retrotransposed in a mouse cell line, suggesting a potential role for L1-based vectors in random insertional mutagenesis.