The effect of heat-killed Pseudomonas aeruginosa (10 serotypes) on antibody formation, macrophage activation and leukemia growth was investigated in relation to the dose injected and to the time and the route of administration. It appeared that intravenous administration of the preparation (10(9) bacteria per ml) was the most efficient at the dose of 0.1 ml since: 1) it increased the number of PFC against SRBC when injected 10 days before the antigen (higher doses and shorter time intervals resulted either in no modification or an significant inhibition of the PFC response; 2) it induced a slight activation of peritoneal macrophages as measured by their cytostatic activity for tumor cells in vitro, when injected 3 or 7 days before testing whereas higher doses were ineffective; 3) it increased the survival time of leukemic mice when administered 2.5 days before the injection of L1210 tumor cells, and higher doses were also effective in this immunoprophylaxis assay. When the subcutaneous route was used, large doses appeared to be the most effective: 1) potentiation of the PFC response was obtained only when 0.5 or 0.2 ml were given 10 days before the antigen; 2) macrophage activation was demonstrated 7 and 10 days after 0.5 ml; 3) leukemia growth was retared when 0.5 or 0.2 ml was injected 2.5 days prior to L1210 tumor cell inoculation and also when 0.2 and 0.1 ml were injected 7 days before tumor cells. No correlation between macrophage activation and the inhibition of tumor growth could be found.