The peripheral blood lymphocytes (PBLs) from 111 cancer patients were isolated and cultured respectively for 23 - 27 days in the medium mainly conditioned by IL-2 and PHA. With ~(125) I-UdR release method, sampling in random way, we examed the cytotoxicities of PBLs and lymphokine-activated-killer (LAK) cells in different culture periods in vitro. The statistic analysis on sufficient data gave the following results: 1. The cytotoxicity against K562 cells increased from 34.78 ?25% of the PBLs to 68.04 ?17.3% of the cells cultured for 8-13 days, afterward, kept about 70% to 23 - 27 days. The constitutional proportion patterns showed that the freshly isolated samples dispersed at a wide range of cytotoxicities (10 - 90%), and that most of the cultured samples ( ~ 85%) concentrated on the range of higher cytotoxicities (50 ~ 95% ) after 8-13 days. 2. The cytotoxicity against Raji cells rose from 8.9 ?8% of the fresh PBLs to 42.1 ?22% of the LAK cells at 8 - 13 days, and maintained about 35% in the following periods. The constitutional proportion patterns of the cytotoxicity against Raji illustrated that all the fresh PBL samples were below 25% of cytotoxicity, and that during the culture, one part of the samples ( ~ 30%) acquired the higher cytotoxicities (50 -90% ), but the other part of the samples ( - 40%) remained at lower cytotoxicities (below 35% ) . The mechanisms behind these phenomena are worth further investigating.