IL-2 kinetics was assessed in mice vaccinated with irradiated syngeneic tumour vaccines carrying an inserted IL-2 gene and producing constitutively IL-2. For comparison, the kinetics of i.v. administered recombinant IL-2 was also examined. During regular time intervals after the vaccination or administration of recombinant IL-2, samples of serum and peritoneal fluid were collected and examined, using CTLL bioassay or its MTT modification. After i.p. administration of irradiated IL-2-producing plasmacytoma (X63-m-IL-2) vaccine, the levels of IL-2 were substantially higher in the peritoneal fluid than in the serum. Both in the peritoneal fluid and in the serum, the IL-2 level was increasing up to 60 min after administration and then it gradually decreased. The last time point when IL-2 was still detectable both in the peritoneal fluid and in the serum was 30 h. Almost identical results were obtained when the IL-2 levels were detected by the conventional CTLL assay, in which DNA synthesis was monitored by 3H-thymidine labeling, and by the isotope-free MTT modification of the CTLL assay, in which the DNA synthesis was monitored by staining. The MTT modification has the advantage of an isotope-free method. Comparison of two different IL-2-producing vaccines, a murine plasmacytoma X63-m-IL-2, with high IL-2 production, and murine sarcoma MC12-IL-2, with low IL-2 production, revealed that whereas after i.p. administration of the high producers, the peak of IL-2 was reached both in the peritoneal fluid and in the serum after 1 h, the administration of low producers gave the peak level of IL-2 later, 5 h after i.p. administration. Comparison of IL-2 levels obtained after i.p. administration of live and irradiated X63-m-IL-2 vaccine revealed that the irradiated vaccine produced both in vitro and in vivo higher amounts of IL-2. As compared to i.p. administration, the kinetics after i.v. administration of the X63-m-IL-2 vaccine was different. The maximum level of recombinant IL-2 was reached 10 min after administration and IL-2 was undetectable after 5 h. When the injections of recombinant IL-2 were repeated, the elimination of IL-2 from the circulation was substantially faster.