The first step in the accurate diagnosis of infectious diseases is to provide the laboratory with adequate specimens for microbiological examination. The sample must be representative of the disease process: samples collected on body surfaces, harbouring saprophytic germ are less reliable than aspiration of closed body areas. A sufficient quantity of infectious material, preferably sampled with a syringe aspiration than with a swab, must be transported rapidly to the laboratory. Finally, close collaboration between the clinician and the microbiologist is essential for appropriate collection of selected specimens and corrected interpretation of the bacteriological results. Gram-stained smear examination of the samples is extremely useful and can provide immediate information for initiation of rational empiric antibiotic treatment. The indications for blood cultures sampling are reviewed. Number of blood cultures and volume of blood per culture are important. Screening of the sputum to discard samples too contaminated by the saliva is mandatory: if only screened samples are considered, a sputum Gram stain is a reliable guide to empiric therapy. Gram stain is also essential in the examination of the cerebrospinal fluid as well as is the detection of bacterial antigens. Cytological and chemical characteristics of this exudate are useful in differentiating bacterial meningitis from other diseases but there is considerable overlap between the laboratory parameters of bacterial meningitis and infections of other etiologies.