1,2-dimethylhydrazine (DMH)-induced colon cancer in Wistar/Furth (W/Fu) rats is analogous in many ways to human colorectal cancer. As part of our attempt to understand the immunobiology of these tumors, we have utilized the recently available monoclonal antibodies W3/25 and OX8 to monitor helper (Th) and suppressor (T,) lymphocyte subpopulations. Normal untreated male W/Fu rats of less than 1 year of age were phenotyped (n = 43). The mean percentage of Th and T, was 42 k l (mean k SEM) and 33 f 1, respectively. The mean Th/T, ratio was 1.3 f 0.1. A Th/T, equal to or greater than 1 is considered "normal" in the W/Fu rat. The DMH-treated rats (20 mg/kg/wk) were evaluated in initial experiments at various intervals after treatment. Rats studied 24 hours after a single DMH injection had no alterations in T cell subsets. Rats studied 28, 32, and 65 weeks after the start of 16 weekly DMH injections were found to have a decrease in the percentage of Th and a relative increase in T,, with Th/T, ratios of 0.6 + 0.2, 0.7 k 0.1, and 0.7 -I 0.1, respectively (each P < 0.01). In a separate experiment in which rats were studied after 4, 8, and 16 weeks of DMH injections, no alterations in T cell subsets were noted. Rats (n = 5 ) studied at 20 weeks after the start of DMH were found to have 41 f 3% Th and 36 f 2% T, and a Th/T, ratio of 1.2 k 0.1. Three of five rats were found to have adenocarcinomas. Four of five rats had Th/T, less than 1. One rat with Th/T, equal to 0.9 had metastatic disease. Rats studied at 25 weeks (n = 8) were found to have more advanced carcinomas (4/8) that were causing obstruction or bleeding in the animal. There was a significant decrease in Th and T, in this group, with 24 +_ 3% and 26 f 3% respectively (P < 0.001). The Th/T, ratio for this group was 0.9 f 0.1 ( P < 0.01). In other experiments, rats were treated with DMH or placebo over a 16-week period and serially bled during and after treatment. No effect of DMH treatment on T cell subsets was noted. Repeated bleeding alone was noted to cause persistent alterations of T cell subpopulation. In conclusion, DMH treatment does not appear to cause changes in peripheral T cell subset changes prior to the onset of tumor development. Even when small tumors are present, the proportions of T h and T, are normal unless the animals are bleeding or are