Protein kinase C (PKC) is an important constituent of the signaling pathways involved in apoptosis. The PKC inhibitor staurosporine induces apoptosis in many cell types. We characterized the role of PKC in the induction of apoptosis in immature rat thymocytes by investigating the effects of staurosporine with those of five analogs. Four of them, the indolocarbazoles CGP 41251 and UCN-01 and the bisindolylmaleimides RO 31-8220 and GF 109203X, possess high PKC-inhibitory specificity and potency, whereas one, the UCN-01 stereoisomer UCN-02, is a weak PKC inhibitor. Apoptosis was examined by flow cytometry, internucleosomal DNA cleavage, and formation of large DNA fragments. Staurosporine, UCN-01, and UCN-02 induced a concentration- and time-dependent increase in apoptosis, whereas neither CGP 41251, RO 31-8220, nor GF 109203X induced apoptosis. The mechanism of induction of apoptosis by staurosporine, UCN-01, and UCN-02 was clearly different from the mechanism that mediates induction of apoptosis by etoposide and dexamethasone, as judged by differential effects of modulators of apoptosis. Staurosporine, UCN-01, and UCN-02 at concentrations of a hundredth to a thousandth of those at which they induced apoptosis, and RO 31-8220 inhibited apoptosis elicited by thapsigargin but not apoptosis caused by dexamethasone or etoposide. The results suggest that (i) UCN-01 and UCN-02 mimic staurosporine as inducers of thymocyte apoptosis; (ii) staurosporine, UCN-01 and UCN-02 share a biphasic effect on apoptosis in rat thymocytes, being inhibitory at low concentrations and stimulatory at high concentrations; and (iii) inhibition of PKC alone is insufficient for induction of apoptosis in thymocytes.