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The growth of bacteria in intravenous solutions and admixtures has been studied under stationary conditions of incubation. All the solutions were inoculated with 100 organisms/ml, incubated at room temperature (27 degrees C) or (37 degrees C), with samples withdrawn at specified time intervals, and plated in quadruplicates. The simple intravenous (i.v.) solutions did not support significant growth (P greater than 0.05) of any of the micro-organisms. Growth in i.v. solutions containing 1% blood was very significant (P greater than 0.05), as demonstrated by the high apparent growth rate constants (K). The ratio of K for beta-lactamase producing bacteria (beta-lac+) over that for non-beta-lactamase producing bacteria (beta-lac-) was significant (P less than 0.05) at 37 degrees C compared to that at 27 degrees C. The higher K values for B. cereus in benzylpenicillin and cefuroxime solutions, respectively, compared to those in antibiotic-free solutions, may be attributable to hydrolysis of the drugs, while the low K values for B. subtilis in the same solutions may be attributed to the inhibitory effects of the drugs. In conclusion, minute quantities of blood in i.v. solution tend to cause bacteria to multiply rapidly. The presence of beta-lactamase producing species might, in addition, hydrolyse susceptible beta-lactam antibiotics which are common additives to i.v. fluids.