The development of apoptosis and the expression of c-fos mRNA were investigated up to 24 hours after treatment (HAT) with 0.2 g/ml of T-2 toxin in Con A-stimulated primary thymocyte cultures prepared from BALB/c mice. Cell viability began to decrease from 1 to 3 HAT, and it was approximately 50% at 6 HAT. The level of cytoplasmic nucleosomes peaked at 3 HAT (about 2 times of control) and decreased thereafter. At 6 HAT, thymocytes showed irregular-shaped or fragmented nuclei. By RT-PCR, the level of c-fos mRNA began to increase within 1 HAT, and maintained a high level through the observation period. Preincubation with BAPTA/AM, a intracellular calcium ion chelator, markedly suppressed the expression of c-fos mRNA and the level of DNA fragmentation induced by T-2 toxin. Although less effective, preincubation with H-7, a protein kinase C (PKC) inhibitor, also depressed the above-mentioned two parameters. These findings indicate that, in mouse thymocyte primary cultures treated with T-2 toxin, c-fos may play an important role in the induction of apoptosis and the increase in intracellular calcium ion may be closely related with the expression of c-fos mRNA. In addition, these findings also suggest that PKC-dependent pathway may be involved in T-2 toxin-induced thymocyte apoptosis.