The effects of retinoic acid (RA) on expression of germ-line transcripts, I alpha C alpha and I gamma 1C gamma 1, and of IgA and IgG1 mRNAs by murine surface IgM-positive B-cells were examined by reverse transcriptase-polymerase chain reaction (RT-PCR). LPS-stimulated B-cells were cultured for 2-3 days in the presence of IL-4 and IL-5 with or without RA. Total RNA was extracted from the cells, and RT-PCR specific for the germ-line transcripts was carried out. RA strongly induced mature germ-line C alpha transcript (I alpha C alpha) at concentrations between 10 and 100 nM. On the other hand, RA completely inhibited IL-4-induced I gamma 1C gamma 1 expression. Significant induction of I alpha C alpha was observed even at a low RA concentration (0.2 nM) in the presence of LPS (1.5-5 micrograms/ml) and without cytokines, and three- to fourfold stimulation of I alpha C alpha induction was seen at 5 nM. I alpha C alpha expression induced by RA (10 nM) and LPS (1.5 micrograms/ml) was not significantly affected by addition of anti-TGF-beta 1 and anti-TGF-beta 2 neutralizing antibodies, although that induced by TGF-beta 1 or TGF-beta 2 was completely inhibited by these antibodies. These results suggest that the major induction pathway of I alpha C alpha was not mediated by active TGF-beta and that RA at physiological concentrations may be involved in IgA isotype switching in vivo in a TGF-beta-independent manner.