A simple technique requiring only 0.2 ml whole blood for measuring the response of lymphocytes in cultures to each of various mitogens and antigens has been elaborated. The response is quantified by comparing the number of lymphocytes with and without a stimulating agent. The increment of cell numbers is given by a cell multiplication index. In healthy subjects PHA induced almost a doubling of the cell numbers in 3 days, i.e. an index of 1.90 +/- 0.38. After 7 days the indexes for PHA, PWM and Con A were 7.25 +/- 4.12, 2.72 +/- 0.65 and 1.81 +/- 0.31, respectively. PPD and Candida-extract induced cell multiplication in skin positive individuals, with indexes ranging from 1.12 to 3.05. In contrast, patients with various severe immune deficiencies showed decreased responses to at least one mitogen, depending on the type of the deficiency. Likewise, skin test negative individuals had no or faint in vitro response to the antigens. The method, which correlated well with the response by a conventional method for incorporation of tritiated thymidine, has a high degree of precision and sensitivity, and should be applicable for routine use.