The hormonally active form of vitamin D3, 1,25-dihydroxy vitamin D3 [1,25-(OH)2-D3; calcitriol], regulates the differentiation and proliferation of epidermal keratinocytes in vitro. MC 903 (calcipotriol) is a novel vitamin D3 analogue which is at least 100 times less potent than 1,25-(OH)-2-D3 in its effects on calcium homeostasis. The present study compared the effects of MC 903 and 1,25-(OH)2-D3 on terminal differentiation and proliferation of cultured normal human keratinocytes. Keratinocytes were grown in McCoy's 5A medium supplemented with penicillin (50 IU/ml), streptomycin (50 micrograms/ml), L-serine (4 X 10(-4) M), and 10% human type AB serum. MC 903, 1,25-(OH)2-D3 or 1 alpha-OH-D3 (10(-12) M--10(-8) M) was added with each feeding when cultures became preconfluent. After incubation for 24 h with D3 vitamins, cultures were extracted for transglutaminase, and the enzyme activity was indexed against DNA content. The activity of transglutaminase, the enzyme responsible for cross-linking the proteins of the cornified envelope, was maximally stimulated by 388% with MC 903 (10(-8) M), by 328% with 1,25-(OH)2-D3 (10(-8) M), and by 27% with 1 alpha-OH-D3 (10(-8) M) compared with vehicle. After incubation for 2 weeks the number of keratinocytes with cornified envelopes had increased by 288% with MC 903 (10(-8) M), by 360% with 1,25-(OH)2-D3 (10(-8) M), and by 149% with 1 alpha-OH-D3 (10(-8) M) compared with vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)